HUALAPAI LIMESTONE MEMBER OF THE MUDDY CREEK FORMATION: THE YOUNGEST DEPOSIT PREDATING THE GRAND CANYON, SOUTHEASTERN NEVADA AND NORTHWESTERN ARIZONA Hualapai Wash—Type locality for the Hualapai Limestone Member of the Muddy Creek Formation. Hualapai Limestone Member of the Muddy Creek Formation: The Youngest Deposit Predating the Grand Canyon, Southeastern Nevada and Northwestern Arizona By WILL N. BLAIR and AUGUSTUS K. ARMSTRONG GEOLOGICAL SURVEY PROFESSIONAL PAPER 111] UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1979 UNITED STATES DEPARTMENT OF THE INTERIOR CECIL D. ANDRUS, Secretary GEOLOGICAL SURVEY H. William Menard, Director Library of Congress Catalog-card No. 79-600142 For sale by the Superintendent of Documents, US. Government Printing Office Washington, DC. 20402 Stock Number 024-001-03222-8 CONTENTS Page Page Abstract 1 Muddy Creek Formation—Continued Introduction 1 Hualapai Limestone Member—Continued Muddy Creek Formation 1 Fossils—Continued Hualapai Limestone Member ----------------------------------------- 1 Ostracodes 8 Distribution 2 Trace fossils 8 Rock types 4 Carbonate rocks 8 Minerals 4 Diagenesis 9 Chert 4 Matrix 9 Fossils 5 Sparry calcite 9 Algal stromatolites ------------------------------------------- 5 Radiaxial fibrous calcite ------------------------------------ 10 Diatoms 7 Age 10 Plant fossils 7 Discussion and conclusions 10 Gastropods 8 References cited 12 ILLUSTRATIONS {Plates follow references] FRONTISPIECE, Hualapai Wash—Type locality for the Hualapai Limestone Member of the Muddy Creek Formation. PLATE 1. Diatoms. 2. Conversion of biogenic silica to cristobalite-rich chert. 3. Lepispheres of bladed cristobalite. 4. Opaline and chert samples. 5. Opaline material. 6. Ostracode valves etched from siliceous matrix. 7. Fossil plants. 8. Algal limestone. 9. Pisolites and microstructures. 10. Fenestrate peloid packstone. 11. Algal oncolites, Grapevine Canyon. 12. Opalized plant stems, Juncus?sp. indet. 13. Peloid packstones. 14—19. Specimens from north of Grapevine Canyon. P age FIGURE 1. Index map of southeastern Nevada and northwestern Arizona 2 2. Photograph showing bedded oncolitic limestone of the Hualapai Limestone Member near Grapevine Wash -------------------- 3 3. Photograph showing westernmost exposure of Hualapai Limestone Member 3 4. Graph of X—ray diffraction patterns of marine rocks 5 5. Photograph showing bedding surface covered with transverse section of algal oncolites 6 6. Diagram and photographs of nest of a large, solitary, ground-nesting bee 9 7. Drawing of radiaxial fibrous calcite cement and inclusion patterns in the Hualapai Limestone Member ------------------------- 10 8. Map showing Miocene and Pliocene embayment of the Gulf of California 11 TABLE Page TABLE 1. K-Ar analytical data and ages 10 $2 HUALAPAI LIMESTONE MEMBER OF THE MUDDY CREEK FORMATION: THE YOUNGEST DEPOSIT PREDATING THE GRAND CANYON, SOUTHEASTERN NEVADA AND NORTHWESTERN ARIZONA By WiLL N. BLAIR and AUcUS'rUS K. ARMSTRONG ABSTRACT The Hualapai Limestone Member of the Tertiary Muddy (Ireek Formation is exposed as erosional remnants in the Lake Mead area of southeastern Nevada and northwestern Arizona and at the base of the Grand Wash (Iliffs in northwestern Arizona. Earlier investigators have considered that this limestone was originally deposited along with mudstone, claystone, and siltstone from interior drainage in a lacus— trine environment. On the basis of an analysis of chert and of the observed occurrence of diatoms typical of estuarine conditions, we in- terpret the limestone as having been formed in marine or brackish waters. The presence of radiaxial fibrous calcite cementing algal on— colites in the Hualapai Limestone Member also indicates early marine sedimentation. Siliceous rocks within the Hualapai Limestone Member exhibit the diagenetic sequence of biogenic opal to cristol)alite to chert. which is characteristic of rocks deposited in a marine environment. The Hualapai Limestone Member was most likely deposited in an estuary at the end of an embayment of the Gulf of California, which extended northward to the edge of the (Iolorado Plateau in the vicinity of Lake Mead. INTRODUCTION This paper presents information on the rock types and environment of the Hualapai Limestone Member of the Muddy Creek Formation. The Muddy Creek For- mation has been considered to be an interior basin de- posit by most workers (Longwell, 1928; Hunt, 1956; Lucchitta, 1967). We believe that there is good evi- dence for a marine or brackish-water origin for the Hualapai Limestone Member. The site for this kind of deposition would have been at the north end of an em- bayment of the ancestral Gulf of California or in an estuary at the north end of the ancestral gulf. MUDDY CREEK FORMATION The Muddy Creek Formation was named the Muddy Creek Beds by Stock (1921, p. 146—147) for exposures between Overton and Logan (now Logandale), Nevada. Longwell (1928, p. 90—96) renamed it the Muddy Creek Formation and (1936, p. 1423) lowered its base to in- clude strata previously assigned to the underlying Horse Spring Formation. The formation is a complex deposit, largely basin and trough type, composed of conglomerate, sandstone, siltstone, mudstone, and crystalline precipitates, It interfingers and is overlain by volcanic flows at several localities. Its uppermost part is distinguished by the Hualapai Limestone Mem- ber, a Widespread flat-lying unit. The Muddy Creek in- cludes abundant salt and gypsum lenses and other in- dicators of aridity (Longwell, 1954). North of the Colorado River, near the head of Boulder Canyon (now under Lake Mead), the Muddy Creek Formation is largely gypsum, which in places is at least 90 m thick. Before the creation of Lake Mead, large beds and plugs of rock salt of Miocene and Pliocene(?) age were ex- posed in part of the Virgin Valley (Longwell and oth- ers, 1965). Moreover, the paleobasin of Red Lake, about 32 km south of Hualapai Wash, contains a min- imum of 1,200 m of evaporites, mainly halite (Peirce, 1976, p. 328). This lake basin appears to have been largely filled by 10 my. ago, before the establishment of the Grand Canyon. Peirce (1976) believes that the halite of Red Lake is the same age as the Muddy Creek Formation. HUALAPAI LIMESTONE MEMBER C. R. Longwell (1936), in his study of the Boulder Reservoir floor of Arizona and Nevada, named the Hu- alapai Limestone for the excellent limestone exposures along Hualapai Wash, its type locality. Lucchitta (1972, p. 1939) considered the Hualapai to be the highest and youngest member of the Muddy Creek Formation. This usage was adopted by Blair (1978). The Hualapai Limestone Member crops out in the northwestern part of Arizona, south of Lake Mead in Mojave County and in southeastern Nevada, north of Lake Mead in Clark County (fig. 1). It is exposed as MESOU/TE 72 KM ‘ HUALAPAI LIMESTONE MEMBER OF THE MUDDY CREEK FORMATION 114° VIRGIN MOUNTAINS L A K E M E A D N A T I O N A L S >< E 8 >. é (I’D 7‘5 . i 40:2 9 a . ‘H 04,9 Q‘OP‘ 9% L . . 36° — o ‘ , v . —§ 0 0—. Nb \\\\\\ J E] ‘ \V [—— - t: . a "a Les - . 3» 'Q- s‘ . \\/3 Q :- ~t‘ ' 1 a" . s E”) § . s““\\ ‘ V) i. - _ _ _ _ s - - .._| (I) 5 1O 15 ZOKILOMETERS SE 1 l I J ilk I DOLAN SPRINGS 40 KM / EXPLANATION '§ “\NEVADA I UTAH *---- - ® 0 ‘\ ® —'—"— CALIFORNIA ‘. \Area ofmap Hualapai Limestone Member of Sample Boundary of Lake Mead ARIZONA Muddy Creek Formation locality National Recreation Area FIGURE l.——Southeastern Nevada and northwestern Arizona showing distribution of Hualapai Limestone Member. erosional remnants astride the present course of the Colorado River at the base of the Grand Wash Cliffs in northwestern Arizona. Ancestral Colorado River gravel is found in siltstone conformably overlying the Hualapai (Blacet, 1975); however, nowhere in the Hualapai is there evidence of a vigorous through-flowing stream such as the pres- ent Colorado River. Therefore, the Hualapai Lime- stone Member probably is one of the youngest deposits formed before the establishment of the Colorado River in its present course from the mouth of the Grand Can- yon. DISTRIBUTION Outcrops of the Hualapai Limestone Member occupy topographic lows that are generally bounded on the west by the Black Mountains and on the north by the Virgin Mountains (fig. 1). The White Hills form an ap- proximate boundary to the south. To the east the Hu- alapai is bounded by the Grand Wash Cliffs, which form a steeply eroded fault scarp across the present course of the Colorado River where it emerges from the Grand Canyon. The best exposure and the type locality of the Hu- alapai Limestone Member is along Hualapai Wash, beginning from the point where the wash joins Lake Mead and extending south for about 9 km (fig. 1). Here the Hualapai is over 240 m thick. A thick section of the Hualapai is also well exposed for about 2 km in steep cliffs along Grapevine Wash, about 8 km south of Pierce Ferry. A third remnant, smaller than the first two, lies ex- posed near Detrital Wash, 8 km southwest of the Lake Mead marina at Temple Bar. Across the lake, smaller and thinner limestone beds cap Delmar Butte, The Temple, and an area just north of Iceberg Canyon. Large exposures of the Hualapai to the south of Lake Mead can easily be reached by a paved road to Temple Bar from Highway 93, and by another paved road through Dolan Springs to Pierce Ferry. This road also departs from Highway 93 but further to the south. Exposures of the Hualapai Limestone Member on the north shore of Lake Mead can be reached by a jeep DISTRIBUTION 3 road from Mesquite, Nev., through the Virgin Moun- tains, a distance of some 72 km. However, a short boat ride across the lake from the Temple Bar marina will bring one to within easy walking distance of the Hu- alapai on the north shore. The easternmost exposures of the Hualapai Lime- stone Member are those south of Pierce Ferry. They extend northward up Gravevine Wash for 12 km, roughly paralleling the Grand Wash Cliffs (fig. 2). Here the Hualapai rests conformably on and interfin- gers with conglomerate of the Muddy Creek Forma- tion. The thickest unit is in Grapevine Wash where it forms rugged and steeply dissected cliffs, in contrast to the softer clayey limestone near the Pierce Ferry airport where the beds thin to about 60 In. The Huala- pai and the conglomerate cover the Grand Wash fault, the exact location of which is unknown. There has been no movement on this fault during the last several hundred meters of deposition of the Muddy Creek. The westernmost exposure of the Hualapai Lime— stone Member is 8 km south of Bonelli Bay (fig. 1) at a point on the east edge of Detrital Wash; it extends eastward from there for about 5 km. The Hualapai at this locality measures over 120 m and dips 5° to the east (fig. 3). The Hualapai overlies 6 In of massive gyp- FIGURE 3.—Westernmost exposure of the Hualapai Limestone Mem» her from locality 1 (fig. 1): (l) marl bed with fossil plants. (2) 8.66- m.y.—old tuff, (3) bed with radioactive material. (4) gypsum bed at the base of the Hualapai Limestone Member. View west. FIGURE 2.—Bedded oncolitic limestone of Hualapai Limestone Member in tributary on west side of Grapevine Canyon near locality 7 (fig. 1). Lake Mead and Grand Wash Cliffs can be seen in distance. View east. 4 HUALAPAI LIMESTONE MEMBER OF THE MUDDY CREEK FORMATION sum of the Muddy Creek Formation. Above the gyp- sum is a 75-m sequence of well-bedded marl, limy clay- stone, and limy mudstone which is well exposed in steep ravines. Interfingering with this sequence are several layers of water-laid volcanic cinders and basalt cobbles. At about 36 m above the base of this sequence, a 150-mm bed of air-fall tuff (fig. 3) can be traced for more than 30 m. The volcanic glass shards in this bed have a K—Ar date of 8.66:2.2 m.y. (Blair and others, 1977). Gypsum lenses up to 50 mm thick and 6 m long occur intermittently near the volcanic debris near the base of the Hualapai. Plant fossils are abundant (fig. 3). At places clumps and bundles of stems can be ob— served in positions of growth, and horizontal root structures weather out of the steep banks. The marl, claystone, and mudstone is overlain by 45 m of dense hard limestone which forms steep cliffs and resistant caps. ROCK TYPES The Hualapai Limestone Member consists of lime- stone interbedded with thin beds of limy claystone, mudstone, and siltstone. The weathered limestone beds of the Hualapai have a predominantly reddish color and form steep cliffs where they are dissected by Hualapai Wash. These cliffs are undercut in many places by erosion of the thin (5—15 cm) softer limy ar— gillaceous interbeds. These argillaceous interbeds thicken and thin laterally over short distances. Scattered chert nodules occur near the top of the Hualapai in a few places. Plant stems replaced by cal- cite and plant leaf impressions in the limestone are common locally. A few casts of small gastropods can be found in the dense limestone layers. Algal structures are abundant in some places. Thin lenses of impure gypsum beds as much as 50 mm thick are sparsely scattered throughout. MINERALS Chert nodules in some beds in the Hualapai Lime- stone Member in the western part of the study area (fig. 1, locs. 2 and 3) are weakly radioactive. The ra- dioactive material consists of thin coatings of carnotite and autunite along bedding planes in the mudstone and claystone. These beds were staked as mineral claims in 1956. Preliminary reconnaissance reports from the Atomic Energy Commission (unpub. data) were submitted in 1955 and 1956 on these claims. The radioactive material ranged from 0.05 to 10.0 mR/hr on the Geiger counter. At the time of these reports, pockets of ore were considered too small and too widely scattered to be of commercial value. Some chert lenses were found to be radioactive. In several places bright yellow secondary uranium minerals can be seen sur- rounding the stems of opalized reed fossils. A few small chert lenses near a prospect pit in the top of the Hu- alapai in the vicinity of the type locality at Hualapai Wash are also weakly radioactive (fig. 1, loo. 6). Chert nodules and thin beds of opal are much more common in the westernmost outcrop than at other ex- posures. Volcanic vents and flows that are associated with these outcrops probably supplied some silica to the waters in which the deposits were formed (Tali- aferro, 1933), but no volcanic particulate material was seen in the siliceous rocks, so this source of silica is thought to be minor. CHERT The diagenetic sequences of siliceous rocks in the Hualapai Limestone Member are similar to opaline chert recovered from a wide variety of recent and an- cient marine sedimentary deposits. We have examined samples of biogenic opal and cristobalite-rich rocks (called chert in this report) of the Hualapai Limestone Member under the scanning electron microscope (SEM) (pl. 5, figs. 2, 4, 6). This study reveals numerous free-growth forms of disordered cristobalite called lep- ispheres (Wise and Kelts, 1972, p. 188). These are mi- croscopic spheres and hemispheres consisting of aggre- gates of cristobalite blades, usually 12—21 um in diameter (pl. 4, figs. 1, 2; pl. 5, figs. 5, 6). The charac- teristic appearance of these minute lepispheres per- mits their identification by the SEM even when only one crystal cluster with a diameter of 2 mm is present (pl. 4, fig. 2). X—ray diffraction patterns for siliceous material in the Hualapai Limestone Member (fig. 4A) are similar to those of marine rocks from opaline claystone of South Carolina (Wise and Weaver, 1973) (fig. 4B) and those from cores taken in the AntarcticOcean (Wise and others, 1972) (fig. 4C). The X-ray patterns for sil- iceous material from the Hualapai are also similar to those produced by Calvert (1971) from deep-sea chert in the North Atlantic, those labeled “common opals” from Australia (Jones and others, 1964), and those from chert of the Monterey Shale of Miocene and Pli- ocene age (Ernst and Calvert, 1969). The sedimentary rocks of the Muddy Creek Forma- tion have been presumed to be derived from fresh to brackish waters (Longwell, 1928; Hunt, 1956; Luc- chitta, 1972). However, the cristobalite—rich rocks of the Hualapai Limestone Member cannot be distin- guished from marine rocks on the basis of SEM studies and X-ray analysis. Examples of marine siliceous rocks that resemble siliceous rocks of the Hualapai Limestone Member are: (1) opaline claystone of Eocene marine shallow shelf strata from Alabama to South FOSSILS ‘ 5 a. Quartz a Crinobalito a. Cristobalite a. Cristobal“. INTENSITY a. Cristobalite uCristobalito a Cristobalite 40 36 32 28 24 20 16 12 DEGREES 20 FIGURE 4.—X-ray diffraction patterns of marine rocks. A, Siliceous material from the Hualapai Limestone Member. B, Opaline clay- stone of South Carolina (Wise and Weaver, 1973). C, Core taken in the Antarctic Ocean (Wise and others, 1972). Carolina (Wise and Weaver, 1973), (2) siliceous rocks of the Monterey Shale deposited in marginal Tertiary basins (Bramlette, 1946; Ingle, 1973; Murata and Lar- son, 1975; Murata and Nakata, 1974), and (3) deep-sea chert from the Antarctic, Pacific, and Atlantic Oceans (Berger and von Rad, 1972; Calvert, 1971; Heath and Moverly, 1971; Weaver and Wise, 1972a, b; Wise and Weaver, 1974). Lepispheres in the opaline part of the Hualapai Limestone Member appear to be larger on the average but not as well formed as those seen in some marine chert (pl. 6, figs. 5, 6). At least one exception can be noted, however, in the micrographs of plate 3, figure 1. The larger lepisphere pictured here is very well rounded and symmetrical. A stereoscopic View shows that it is probably attached to the cavity walls by the small quartz crystals near its surface. The scanning electron micrograph of plate 5, figure 4, shows clean cristobalite crystal blades with scal— loped and serrated edges. Wise and Weaver (1973) noted cristobalite blades with similar serrated-edge patterns from marine opaline claystone of Alabama and from deep-sea chert. Diatomaceous biogenic opal (opal-A of Jones and Segnit, 1971) is one of the most important sources of silica in pelagic sediment of recent oceans (Riedel, 1959). In fact, biogenic opal is the silica source for most deep-sea chert (Wise and others, 1972; Davies and Supko, 1973; Wise and Weaver, 1974). The evidence from the siliceous rocks in the Hualapai Limestone Member indicates that diatom frustules were the ini- tial most important contributor to its nodular chert. This evidence reinforces the concept of the diagenetic sequence of biogenic opal (opal-A) to cristobalite (opal- CT) to chert. The authors subscribe to this kind of ma- turation sequence as advocated by Heath and Moberly (1971), Weaver and Wise (1972b), Berger and von Rad (1972), and Wise and Weaver (1974). FOSSILS ALGAL STROMATOLI'I‘ES The Hualapai Limestone Member contains circular to irregular oval objects from less than a millimeter to 10 cm in diameter (fig. 5). In thin section these show irregular banding (pl. 9, figs. 2, 4, 6; pl. 10, figs. 1—6, pl. 11, figs. 1—6, pl. 13, figs. 2—6; pl. 17, fig. 1; pl. 18, figs. 1—6). These we consider to be algal oncolites. On- colites are unattached stromatolites with encapsulat- ing laminae (Walter, 1976, p. 691). The specimens from the Hualapai Limestone Member (pl. 9, figs. 2, 4, 6; pl. 10, figs. 1—6) are similar in shape and size to Bradley’s (1929, pls. 35, 42) digitated algae from the Green River Formation of Utah. Johnson (1961, pls. 132—133) shows similar specimens from the Green River Formation of Wyoming and Colorado. Specimen 892D (pl. 9, figs. 2, 4, 6) of a digitate on- colite, is composed of irregular dark-brown and light bands of calcite which form botryoidal intergrowths. The narrower dark bands are composed of 3—5 pm cal- cite rhombs, whereas the lighter bands are formed by long slender calcite rhombs 50—100 mm long (pl. 9, fig. 6 HUALAPAI LIMESTONE MEMBER OF THE MUDDY CREEK FORMATION FIGURE 5.—Outcrop of bedding surface covered with transverse section of algal oncolites, typical of much of the thick Hualapai Limestone Member. Photograph taken in first canyon north of Grapevine (Ianyon. Coin is 21 mm across. 6). Algal oncolites contain detrital quartz grains and fragments of volcanic rocks. The matrix that sur- rounds the algal oncolites is composed of detrital grains of quartz and volcanic rock in peloid-lime mud- stone. These oncolites commonly have as their center or at their bases grains or pebbles of volcanic rocks or micritic mudlumps. Gebelein (1976) states: The environment of formation of oncolites at J oulters Cays, Baha- mas, is similar to that for attached stromatolites, except that peri- odic turbulence, causing initial detachment of the forms, is higher. In the J oulters, this increase in turbulence corresponds to a decrease in depth. Maximum formation occurs in depths less than 1 m. Con- centrations of 10—100 oncolites per m2 are common in the shoal areas. Lags of oncolites carpet the floor of some tidal channels which cut through the shoals. Algal oncolite specimen 846A (pl. 8, figs. 1—7) is a digitated form, with individual branches up to 1 mm in diameter. In transverse section the dark and light banding is somewhat circular, and in longitudinal sec- tion, the banding shows a distinct arched growth pat- tern. The long axes of the calcite crystals are tangen- tial to the circular bands of the algae. Algal oncolites in transverse section, with crossed nicols, show a pseudo-uniaxial cross. The algal oncolites from Grapevine Canyon (pl. 11, figs. 1—6) are up to 3 cm in diameter, are spherical in shape, and retain void spaces between oncolites. Voids within the oncolite and between growth bands are lined with cryptocrystalline quartz (opal) (pl. 11, figs. 5, 6). Some algal oncolites (pl. 13, figs. 2, 4, 6) contain within their bands appreciable amounts of detrital quartz grains. Algal oncolites commonly are crusts on compound micrite mudlumps, formed of detrital quartz and micritic peloids (pl. 17, fig. 1). These algal crusts show a tendency for laminations developed on one side of the mudlump, suggesting growth upward (pl. 18, fig. 2). Colloform algal stromatolites formed by digitate fab- ric occur in the Hualapai Limestone Member at Grapevine Canyon. Individual algal branches are 1—3 mm in diameter and may be up to 20—30 cm tall (pl. 8, figs. 1—6). Hoffman (1976) and Playford and Cock- bain (1976) report colloform mat structures from the sublittoral shelf of Hamelin Pool, Shark Bay, Aus- tralia. These recent stromatolites have shapes similar to colloform stromatolites from the Hualapai Lime- stone Member. Hoffman’s (1976, p. 269, figs. 6d, 6e) photographs of recent colloform tilted stromatolites are morphologically similar to the colloform stroma- tolites in the Hualapai. The Hamelin Pool colloform stromatolites live in a semiarid climate and subtidal FOSSILS 7 environment with salinity nearly twice that of normal seawater. Hoffman states that on the sublittoral floor, stromatolites with digitate columnar internal struc- ture occur beneath ovoid patches of colloform mats. These colloform stromatolites are buried in pelleted lime mud. The Hamelin Pool fauna has low species diversity because of the hypersaline condition, and al- gae-consuming organisms are largely absent (Playford and Cockbain, 1976). As a result algal stromatolites flourish on the sublittoral and intertidal platforms. In Hamelin Pool lithification of stromatolites has taken place in both the intertidal and subtidal environments. Laminations are only crudely developed or absent in many Hamelin Pool stromatolites, and fenestrate (birdseye) fabrics are characteristic. Shinn (1968) recognized two kinds of birdseye struc- tures in recent and ancient carbonate rocks: (1) planar isolated vugs 1—3 mm high by several millimeters wide and (2) isolated bubblelike vugs 1—3 mm in diameter. Both types are generally filled with calcite or anhy— drite. Shinn’s study of more than 100 cores and samples of recent carbonate sediments showed that birdseye voids are preserved in supratidal sediments (sediments de- posited above normal high-tide level), sometimes in intertidal sediments (sediments deposited between normal and high-tide level), sometimes in intertidal sediments (sediments deposited between normal high and normal low tide), and never in subtidal sediments (sediments permanently below water). Both kinds of birdseyes are particularly abundant in supratidal do- lomitic sediments. Playford and Cockbain’s (1976, p. 410, fig. 11A) ex- ample of a well-lithified intertidal stromatolite from Hamelin Pool, showing the characteristic fenestral fabric in weakly laminated pellet wackestone, is sim- ilar to fenestrate pellet packstone of the Hualapai Limestone Member (pl. 10, figs. 1—6; pl. 13, figs. 1, 3, 5; pl. 15, figs. 1, 6). DIA'I‘OMS Diatoms were recovered by J. Platt Bradbury, of the US. Geological Survey, from opalized rock samples we collected near the top of the Hualapai Limestone Mem- ber at localities 1 and 4 (fig. 1). He identified the spec- imens from locality 1 as Scoliopleura sp., Nitzchia sp., Mastogloia cf. M. aquilegiae, Rhopalodia gibberula, Navicula cf. N. radiosa tenella, Amphora or Cymbella sp., and Denticula sp. White opal from an outcrop of the Hualapai Lime- stone Member 6 km southeast of Temple Bar (fig. 1, Ice. 4) was studied by the authors under the scanning electron microscope. This study revealed the existence of exceptionally well preserved diatoms imbedded in the opal. Scanning electron micrographs of these dia- toms are presented on plate 1. Bradbury has identified these species (written commun., 1976) as Navicula cf. N. cuspidata (pl. 1, fig. 1) which is widespread in al- kaline freshwater habitats. NavicuIa hanphiIa (pl. 1, fig. 2) is a true brackish-water diatom that can tolerate considerable variations in salinity. This diatom is characteristic of inland lakes and marshes of moderate salinity but it is also found in coastal (marine) envi- ronments. Melosira moniliformis? (pl. 1. figs. 5, 6), Amphora hyalina (pl. 1, fig. 4), and Amphora arcus var. sulcata (pl. 1, fig. 3) are marine or brackish-water diatoms that are common in coastal littoral habitats. Frequently Amphora species occur in estuarine environments. According to Bradbury, Melosira monfliformis?, Am- phora arcus var. sulcata, and Amphora hyalina seldom characterize inland saline aquatic environments. Their presence suggests that a coastal, perhaps es— tuarine, environment existed when the diatomaceous sediments were deposited. The diatoms suggest that the Hualapai Limestone Member is not older than Neogene. The diatom family Epithemiaceae which includes the genera Denticula and Rhopalodia is thought not to be older than Pli- ocene. PLANT FOSS I LS The remains of grasses, reeds, and rushes are the most abundant macrofossils found in the Hualapai Limestone Member. These can be observed as layered horizontal mats entirely replaced by limestone. Others form vertical hollow calcite tubes, having inner sur- faces lined with drusy quartz and exteriors encrusted with travertine. In at least one locality (fig. 1, 10c. 1) the reed stem structures have been perfectly replaced by opal, preserving the cell walls (pl. 12, figs. 1—6). Molds and casts of the plants resemble Juncus (Jun- caceae, the rush family), but they have not been iden- tified with certainty. Some of the plant fossils with hollow stem structures look like scouring rushes (Equisetacae): these rushes are now represented only by a single genus, Equisetum (Robinson and Fernald, 1908, p. 51; Scott, 1962, p. 13). The stems usually have hollow internodes with a coat- ing of silex, which is the zone that is normally pre- served. Equisetum is a silica accumulator plant that takes silica into its system from the soil. This silica is precipitated around the cell walls in the form of plant opal (Lovering and Engel, 1967, p. B15). When the plant dies, the silica then is either dispersed in the form of phytoliths, or, if the plant is buried before this dispersal, its opalized structure can be preserved in the 8 HUALAPAI LIMESTONE MEMBER OF THE MUDDY CREEK FORMATION rock record. The amount of silica contributed to the Hualapai Limestone Member in this manner, however, was probably minor. GAS'I‘ROPODS Gastropod molds and steinkerns were collected at several localities in the Hualapai Limestone Member and were studied by J. H. Hanley (unpub. data). One poorly preserved complete steinkern of a sinistral gas- tropod may represent the Physidae. This specimen was one of several collected by the authors from Hualapai Wash (fig. 1, loc. 5). Twenty incomplete external molds and four incomplete steinkerns collected from locality 4 (fig. 1) may represent a single species of F Iumnicola Stimpson. FIumnicoIa is known from two other local- ities in the Muddy Creek Formation in Clark County, Nev.: (1) USGS Cenozoic locality 22250, Dry Lake quadrangle. El/z sec. 35, and W1/2 sec. 36, T. 19 S.', R. 63 E., carbonate facies. Collected by C. R. Longwell, 1960, and (2) USGS Cenozoic locality 22252 (LID—~58). SW1/2 sec. 26, T. 19 S., R. 63 E., calcareous facies. Col— lected by W. H. Hays, 1958. The Hydrobiidae, which includes FIumnicoIa, are herbivorous, ingesting algae and algal detritus, but diatoms constitute the principal food source for some species. Hydrobiids occur in aquatic habitats of many sizes, from creeks and springs to the Great Lakes. Some species occur in brackish and marine habitats. These gastropods live on a variety of substrates in- cluding mud, sand, gravel, and aquatic plants. The wide-ranging habitats of the fossil gastropods of the Hualapai Limestone Member preclude their use as indicators of the paleoenvironment. OSTRAC()1)ES Ostracodes are locally abundant in the Hualapai Limestone Member. Several ostracodes were observed in thin section. None could be definitely classified, but at least one individual resembles a species of Candona (J. P. Bradbury, written commun., 1976). Candona is typical of the freshwater forms, which are character- ized by smooth and monotonous surface features (Ben- son, 1961). Species of Candona are found in all types of freshwater environments, ranging from deep water to marshes. They are nonswimmers, usually burrow- ing in the mud. Over 100 species of Candona have been described (Van Morkhoven, 1963). Its eggs not only require no care but can stand desiccation for long per- iods of time. On the other hand, the eggs of marine ostracodes cannot survive any drying out whatsoever (Kesling, 1961). Ostracode remains from limestone near Grapevine Canyon appear relatively abundant under the scan- ning electron microscope (pl. 6, figs. 1, 3), and in thin section (pl. 17, fig. 4). These were too poorly preserved to be classified, but they also resemble Candona. Opaline rocks containing reed fossils from an area 12 km southwest of Temple Bar (fig. 1, 100. 1) were etched for 24 hours in a solution of one part hydro- fluoric acid to nine parts of water. Many silicified ostra- code valves were freed from the matrix, but these were also poorly preserved (pl. 6, figs. 1, 3). TRACE FOSSILS Distinctive flask-shaped casts about 2 cm in diameter occur in the Hualapai Limestone Member 12 km southwest of Temple Bar marina. These distinctive fos- sils weather out from clayey limestone beds in the side of a road cut. These casts invariably have much the same shape and dimensions. They are alined perpendicular to the bedding planes of the sediments within a few centi- meters of each other, probably much in the same po- sition as they were formed in life. They are calcareous thin—walled tubular structures, broken at the upper end and closed in a bulbous form at the lower end. The open end tapers inward slightly to form a neck, which then enlarges downward to produce the ovoid base. Jerome G. Rozen, Jr., of the American Museum of Natural History, has suggested (written commun., 1976) that these fossils are cells of aculeate Hymenop- tera (bees and wasps). Probably these casts are re- mains of nests of a large, solitary, ground-nesting bee. That is, they are chambers into which the female bee carried pollen and nectar and in which she laid her eggs. The narrow rounded ends of the cell in these chambers seem to be unusual and remind Rozen of a bee of the southwest, PtiIogIossa, which he has stud- ied. This genus belongs to a primitive family of bees which could easily have been here 8.5 m.y. ago. Nest patterns of solitary bees vary from species to species and genus to genus. They usually consist of a burrow descending into the ground with branching passages and with cells at the ends of the branches (fig. 6). CARBONATE ROCKS Dunham’s (1962) carbonate rock classification is used in this report. The Hualapai Limestone Member, as studied in the stratigraphic section near Grapevine Canyon (fig. 2), is composed predominantly of micritic limestone. The only significant fossil bioclasts here are algal oncolites, rare fragments of ostracodes, and mol- lusks. The limestone is porous, owing to incomplete sparry calcite cementation between micrite mud- lumps, algal oncolites, and pellets. CARBONATE ROCKS 9 Su rface Intermediate- sized larva «‘73:. a SmallilarVa:% " “a; a.) Large “.3». I feeding i 7 ' ~ larva FIGURE 6.—Nest of a large, solitary, ground-nesting bee with photo- graphs of fossil casts for comparison. Modified from Rozen (1974). Terrigenous admixtures are abundant in the micri- tic carbonate rocks of the Hualapai Limestone Mem- ber. They are silt- and sand-size quartz grains and fragments of volcanic rocks (pl. 9, fig. 2; pl. 10, fig. 1; pl. 12, figs. 4—6; pl. 13, figs. 2, 4, 5; pl. 16, figs. 2—6; pl. 18, figs. 1, 2; pl. 19, figs. 1, 2, 4). DIAGENESIS For the purpose of this paper, diagenesis will include all changes that sediments undergo from the time of deposition until the present (Wood and Armstrong, 1975, p. 15). One of these changes is lithification,‘ the change from unconsolidated sediment to consolidated sediment by compaction, cementation, and pressure ‘ "solution. MATRIX Matrix is usually composed of carbonate mud (mi- crite) and is dominantly of biologic origin. The sepa- ration of grains and matrix is to some extent a function of the scale of observation, but in practice a tendency toward a bimodal size distribution in micritic lime- stone makes the distinction easy. Individual crystals of micrite have an upper size limit of 3—4 pm (Ba- thurst, 1959). Choquette and Pray (1970, p. 247) define interparticulate porosity for between-particle porosity in sedimentary carbonates. It can be used for lime mud and micrites as well as for coarser clastic carbonate. Recent carbonate mud is largely bioclastic in origin, whether originally aragonite (described by Cloud, 1962) or calcitic in composition (Davies, 1970). The only known examples of inorganic precipitation of cal- cium carbonate are in the lagoon of Abu Dhabi (Kins- man and Holland, 1969) and Trucial Cast, Persian Gulf (Evamy, 1973), and the lower to middle intertidal zone of Shark Bay, Australia (Logan, 1974). Stockman, Ginsburg, and Shinn (1967) have shown that great quantities of aragonite mud can be produced by codi- acean algae in Florida Bay and can more than account for the aragonite mud present. The abundant micrite (pls. 14—18) in the Hualapai Limestone Member is probably the result of organic activity by codiacean and other algae. The absence of fossil bioclasts, other than algal oncolites and a rare ostracode, precludes significant formation of micrite derived from larger shelled invertebrates. SPARRY (IALCl'l‘E Primary pore space and interparticle space are still present within the Hualapai Limestone Member. Two distinct phases of sparry calcite cement are found within the interparticle space. The first—phase cement forms a thin fringe of relatively constant thickness of 40 um around the algal oncolites and peloids (Shinn, 1969) (pl. 10, figs. 2, 3, 5; pl. 13, figs. 1, 5, 6; pl. 18, fig. 6). These are stubby calcite crystals with pyramidal terminations growing at right angles to the grain mar- gins. Bathurst (1971, p. 432) shows that the first gen— eration of cement crystals, besides being smaller, are commonly scalenohedral in habit (dogtooth spar) and are preserved under the later overgrowth of second- generation rhombohedra. The sharp distinction in size and habit between the earlier and later crystals may indicate a time interval between formation of the two Sizes. 10 The source of the calcium carbonate necessary for the complete cementation implies that the second- phase cement is from another source. Bathurst (1971, p. 457) suggested that neither solution of local aragon- ite nor pressure solution will provide sufficient volume to nearly obliterate the intergranular space. Within the Hualapai Limestone Member a very large propor- tion of the originally aragonitic micrite and bioclasts has gone into solution by vadose waters and has been redeposited as void-filling sparry calcite. This process could provide the large amount of sparry calcite ce- ment seen in the Hualapai (pl. 10, figs. 1—6; pl. 13, figs. 1—6; pl. 15, figs. 1—6; pl. 16, figs. 1—4; pl. 17, fig. 4; pl. 18, figs 1—6). RADIAXIAL FIBROUS CALCITE Limestone specimens from the section south of Grapevine Canyon have numerous cavities filled with fibrous radiaxial calcite (pl. 15, figs. 4, 5; pl. 16, figs. 4, 5; pl. 19, figs. 1—6). The radiaxial cement occurs as void-filling material between algal oncolites, in lime mudlumps, and in fenestrate voids in the lime mud- stone. The radiaxial fibrous calcite typically contains numerous zones of inclusions (fig. 7) The inclusion }lmpurity zones _,.,_|nclusion zone par- allel to substrate }Micrite substrate FIGURE 7.—Radiaxial fibrous calcite cement and inclusion patterns in the Hualapai Limestone Member. Greater part of mosaic is com- posed of fibrous crystals, long axes oriented normal to substrate. Marginal zones exhibit decrease in crystal size toward substrate. HUALAPAI LIMESTONE MEMBER OF THE MUDDY CREEK FORMATION zones may be parallel to the substrate. The most com- mon type of inclusion is scalenohedral forms that may reveal the positions of former calcite crystal faces (pl. 18, fig. 1). Kendall and Tucker (1973) in the detailed discussion of the origin and environmental significance of radiax- ial fibrous calcite indicate that it is a replacement after acicular cement. They state, “* * *acicular cement ap- pears to be largely restricted to marine, early diage- netic environments. Consequently, formation of ra- diaxial fibrous mosaics also occurs during early diagenesis.” AGE An' air-fall tuff 15 cm thick, that is completely within the lower part of the Hualapai Limestone Mem- ber, crops out at locality 1 (fig. 1) 12 km southwest of Temple Bar. The glass shards of this tuff have yielded a K-Ar date of about 87:22 my. (Blair and others, 1977; see table 1), considered to be late Miocene. The maximum and minimum age limits of the Hu- alapai Limestone Member are not known. However, a basalt in the lower part of the exposed section of the Muddy Creek Formation 6 km southeast of Temple Bar (fig. 1, loc. 4) underlies the Hualapai. This basalt has been dated using the K-Ar method by McKee (un- pub. data; see table 1) at about 10.9 m.y. A basalt flow at Sandy Point (fig. 1) separated from the underlying Muddy Creek Formation by fluvial gravel has been dated at 3.79:0.46 m.y. by Damon (Damon and others, 1978, p. 102). DISCUSSION AND CONCLUSIONS The Hualapai Limestone Member, considered by most workers to be freshwater in origin, has many sim- ilarities to marine deposits (Blair, 1978). The water which deposited the Hualapai was most likely con- TABLE 1.—K-Ar anaIyticaI data and ages of samples [Analysis by E. H. McKee, U.S. Geological Survey] K20 “Ar”ld MlAr”Id Age1 Age2 Locality Sample (percent) (mole/g) (percent) (my) (my) Source (fig. 1, loc. 1) Basaltic shards in air-fall tuff, lower part of Hualapai Limestone Blair and others, Member ------------------------ 2.462 3.07633x10-ll 7.1 8.44+2.23 8.66:2.2 (1977). (fig. 1,10c. 4) Basalt below Hualapai Lime- 1.045 E. H. McKee (unpub. stone Member ---------------- 1.067 1.66192 X 10!1 17.1 10.62 t 1.10 10.90 t 1.1 data). lAges based on old constants for comparison with published ages. Xe = 0.58 ><10-1°yi—l AB = 4,72 X IOWyr1 CM 5-10“ I ‘OK/Kmm = 1.22 X 10-4g/g 2Ages based on new constants. K: = 0.581 X 10'10yrl M} : 5.96x lo-loyrl “WK/Kw,“ : 1.17 X 10'4g/g DISCUSSION AND CONCLUSIONS 1 1 nected with the upper reaches of the Gulf of California in late Miocene time and was hence the upper estua- rine part Of the ancestral gulf. Marine beds of the Pli- ocene Bouse Formation (Metzger, 1968), which were deposited in the ancestral Gulf of California, have been mapped as far north as the Parker area, about 180 km south of the southernmost exposures of the Hualapai Limestone Member (fig. 8). This embayment no doubt reached further north. This arm of the ancestral Gulf of California (Karig and Jensky, 1972) probably ex- tended to the foot of the Grand Wash Cliffs as a shal- low estuary. Marine, brackish, and freshwater fossil assemblages occur together in the Bouse Formation, a phenomenon that puzzled Smith (1970) when she studied the fossils Of that formation. She speculated, however, that such NEVADA l f I l l CALIFORNIA \ Southern limit of Hualapai E: \ Limestone Member of j; \ Muddy Creek Formation '55_ 36° - \ ' Postulated ancestral Gulf of California (this report) Approximate northern limitof Bouse Formation and Miocene and Pliocene Gulf of California v6 70 Ca EXPLANATION “o §§§ Hualapai Limestone Member of Muddy Creek Formation Postulated ancestral Gulf of California Bouse Formation and ancestral Gulf of California Boundary of Colorado Plateau i WYOMING —_- fi_———-—-———--—’ / ’ / ” :\ /' l \ COLORADO / UTAH 3% / ‘9 l i \ COLORADO ' \ PLATEAU I \ l l ___ ____}p ‘-—----—- ——’d ‘ 03A MEXICO 0 200 KILOMETERS l_—l FIGURE 8.—Miocene and Pliocene embayment of Gulf of California. Modified from Schuchert, (1955); Metzger, (1968); and Smith, (l970). 12 HUALAPAI LIMESTONE MEMBER OF THE MUDDY CREEK FORMATION an assemblage might be due to mixing of fossils by sed- imentary processes. The restricted number of species of foraminifers found in the northern areas of the Bouse is typical of modern bays and estuaries. Fresh- water mollusks and ostracodes are found with the for- aminifers, but the presence of marine clams and snails classifies the Bouse Formation as marine. Several lines of evidence indicate that as the Huala- pai Limestone Member was being deposited, seawater was mixing with fresh water in an estuary: (1) Marine coastal and estuarine diatoms occur together with freshwater forms in the upper part of the Hualapai Limestone Member (pl. 1, figs. 1—6). (2) Freshwater and marine fossil assemblages are mixed in the Hualapai, as in the Bouse Formation to the south. A logical source for the freshwater types would be from a river (the ancestral Colorado River) entering from the north. (3) Existing data suggest that the diagenesis of biogenic opal conversion to chert occurs pre- dominantly in marine environments. How- ever, Oehler (1973) experimentally pro- duced lepispheres in water with only about 700 ppm of Na*; this concentration ap- proaches fresh water. This study may show that lepispheres can also form an estuarine environment. (4) The mass of limestone (over 100 kma) indi- cates a large volume of water containing calcium, probably seawater. (5) Radiaxial fibrous calcite cement fills voids be- tween algal oncolites and micritic litho- clasts in the Hualapai Limestone Member. Radiaxial fibrous calcite cement is consid- ered to be largely restricted to marine en- vironments. (6) Stromatolites from the Hualapai Limestone Member are typical of forms found in shal- low marine environments (fig. 6). (7) An air-fall tuff in the lower part of the Hu- alapai was dated at about 8.7 my (table 1). The hypothesis that would account for all this data is that a shallow, marshy, marine estuary existed in late Miocene time, extending up the area of the lower Colorado River to the base of the Colorado Plateau where the river now emerges from the Grand Canyon. The elevation at this point was then at sea level, some 420 m lower than it is now. A large slow-moving river (the ancestral Colorado River?) entered the estuary from some unknown direction. REFERENCES CITED Bathurst, R. G. C., 1959, Diagenesis in Mississippian calcilutites and pseudobreccias: Jour. Sed. Petrology, v. 29, p. 365—376. 1971, Carbonate sediments and their diagenesis: Amsterdam, Elsevier, 620 p. Benson, R. H., 1961, Ecology of ostracode assemblages, in Moore, R. 0., ed., Treatise on invertebrate paleontology, Pt. Q, Arthropoda 3: Lawrence, Kans., Geol. Soc. American and Kansas Univ. Press, p. Q56—Q63. Berger, W. H., and Rad, Ulrich von, 1972, Cretaceous and Cenozoic sediments from the Atlantic Ocean, in Hayes, D. 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H., 1961, Limestone building algae and algal limestones: Golden, C010,, Colorado School Mines, 297 p. Jones, J. B., Sanders, J. V., and Segnit, E. R., 1964, Structure of opal: Nature, v. 204, p. 990—991. Jones, J. B., and Segnit, E. R., 1971, The nature of opal. I. Nomen- clature and constituent phases: Geol. Soc. Australia J our., v. 18, p. 56—68. Karig, D. E., and Jensky, W., 1972, The proto-Gulf of California: Earth and Planetary Sci. Letters 17, p. 169—174. Kendall, A. C., and Tucker, M. E., 1973, Radiaxial fibrous calcite—— A replacement after acicular carbonate: Sedimentology, v. 20, p. 365—389. Kesling, R. V., 1961, Reproduction of ostracoda, in Moore, R. 0., ed., Treatise on invertebrate paleontology, pt. Q, Arthropoda 3: Law- rence, Kans., Geol. Soc. America and Kansas Univ. Press, P. A17—A19. Kinsman, D. J. J ., and Holland, H. D., 1969, The co-precipitation of cations with CaCOa. The co-precipitation of S1” with aragonite between 16° and 96°C: Geochim. et Cosmochim. 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E., 1976, Modern algal stromato— lites at Hamelin Pool, a hypersaline barred basin in Shark Bay, Western Australia, in Walters, M. R., ed., Developments in sedimentology 20, Stromatolites: Amsterdam, Elsevier, p. 389— 411. Peirce, H. W., 1976, Tectonic significance of Basin and Range thick evaporite deposits: Arizona Geol. Soc. Digest, v. 10, p. 325—339. Riedel, W. R., 1959, Siliceous organic remains in the pelagic sedi- ments: Soc. Econ. Paleontologists and Mineralogists Spec. Pub. 7, p. 80—91. Robinson, B. L., and Fernald, M. L., 1908, Grey’s new manual of botany, a handbook of the flowering plants and ferns of the cen- tral and northeastern United States and Canada: New York, America Book Co., 926 p. Rozen, J. G., Jr., 1974, Nest biology of the Eucerine bee Thygater analis (Hymenoptera, Anthoporidae): Jour. New York Entomol. Soc, v. 82, no. 4, p. 230—234. Schuchert, Charles, 1955, Atlas of paleogeographic maps of North America: New York, John Wiley & Sons, 177 p. Scott, D. H., 1962, Studies in fossil botany (3d ed.): New York, Haf- ner, v. 1, 434 p. Shinn, E. A., 1968, Practical significance of birdseye structures in carbonate rocks: Jour. Sed. Petrology, v. 38, no. 1, p. 221—224. 1969, Submarine lithification of Holocene carbonate sedi- ments in the Persian Gulf: Sedimentology, v. 12, p. 109—144. Smith, P. B., 1970, New evidence for Pliocene marine embayment along the lower Colorado River area, California and Arizona: Geol. Soc. America Bull., v. 81, p. 1411—1420. Stock, Chester, 1921, Late Cenozoic mammalian remains from the Meadow Valley region, southeastern Nevada [abs]: Geol. Soc. America Bull., v. 32, p. 146—147. Stockman, K. W., Ginsburg, R. N., and Shinn, E. A., 1967, The pro- duction of lime mud by algae in south Florida: Jour. Sed. Pe- trology, v. 37, p. 633—648. Taliaferro, N. L., 1933, The relation of volcanism to diatomaceous and associated siliceous sediments: California Univ. Pubs. Geol. Sci., v. 23, no. 1, 56 p. Van Morkhoven, F. P. C. M., 1963, Post-Paleozoic ostracoda: Am- sterdam, Elsevier, v. 2, 478 p. Walter, M. R., 1976, Glossary of selected terms, Appendix I ofWal- ter, M. R., ed., Developments in sedimentology 20, Stromato- lites: Amsterdam, Elsevier, p. 687—692. Weaver, F. M., and Wise, S. W., 1972a, Chertification phenomena in Antarctic and Pacific deep sea sediments—A scanning elec- tron microscope and X-ray diffraction study: Am. Assoc. Petro- leum Geologists Bull. 56, p. 1905. 1972b, Ultramorphology of deep sea cristobalitic chert: Na— ture Phys. Sci., v. 237, p. 56—57. Wise, S. W., Buie, B. F., and Weaver, F. M., 1972, Chemically pre- cipitated sedimentary cristobalite and the origin of chert: Eclo- gae Geol. Helvetiae, v. 65/1, p. 157—163. Wise, S. W., and Kelts, K. M., 1972, Inferred diagenetic history of a weakly silicified deep sea chalk: Gulf Coast Assoc. Geol. Soc. Trans, v. 22, p. 177—203. 14 HUALAPAI LIMESTONE MEMBER OF THE MUDDY CREEK FORMATION Wise, S. W., and Weaver, F. M., 1973, Origin of cristobalite-rich Ter- Wood, G. V., and Armstrong, A. K., 1975, Diagenesis and stratig- tiary sediments in the Atlantic and Gulf Coastal Plain: Gulf raphy of the Lisburne Group limestones of the Sadlerochit Coast Assoc. Geol. Soc. Trans, v. 23, p. 305—323. Mountains and adjacent areas, northeastern Alaska: US. Geol. ——1974, Chertification of oceanic sediments: Internat. Assoc. Se- Survey Prof. Paper 857, 47 p. dimentology Spec. Pub. 1, p. 301—326. PLATES 1—19 Contact photographs of the plates in this report are available, at cost, from US. Geological Survey Library, Federal Center, Denver, Colorado 80225 PLATE 1 FIGURES 1—6. Scanning electron micrographs of diatoms found together in opaline rock in the Hualapai Limestone Member. The diatoms range in habitat from freshwater to marine. All are from locality 4 (text fig. 1). 1. Navicula cf. N. cuspidata. Diatom widespread in freshwater habitats. 2. Navicula halophila. This is a true brackish-water diatom. 3. Amphora arcus v. suIcata. Marine or brackish-water diatom. 4. Amphora hyaIina. Marine or brackish-water diatom. 5. Melosira moniliformis? Interior view. Marine or brackish-water diatom. 6. Melosira moniliformis? PROFESSIONAL PAPER 1111 PLATE 1 5 0 30pm 6 0 30pm —' L———_———l DIATOMS PLATE 2 FIGURES 1—6. Scanning electron micrographs showing‘progressive conversion of biogenic silica to cristobalite-rich chert. l. 2. 3. 9"!“ Slightly altered diatom frustules. Diatom frustules filled with bladed cristobalite. Diatom frustule partly filled with bladed cristobalite. Remainder is filled with chert. Altered diatoms which are gradually losing their identities. Vestiges of diatom frustules merging in chert. Cristobalite chert with textures and fossil evidence obliterated. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1111 PLATE 2 r34 0 10pm O 30,um |___.____l L____—J 0 30m 0 20m L—-—’ L———-—_—l CONVERSION OF BIOGENIC SILICA TO CRISTOBALITE-RICH CHERT PLATE 3 FIGURES 1, 2. Scanning electron micrographs showing lepispheres of bladed cristobalite in chert sample from locality 4 (text fig. 1). 1. Stereopair set at 65 mm interocular distance. Well-formed lepisphere was growing in cavity in chert. Small embryonic lepispheres are scattered about cavity. Note small quartz crystal penetrating sphere at its upper right quadrant. 2. Enlargement of embryonic lepispheres indicated by arrow in figure 1. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1111 PLATE 3 2 0 1pm I.___..__._l LEPISPHERES OF BLADED CRISTOBALITE PLATE 4 FIGURES 1—6. Scanning electron micrographs and elemental distribution maps of opaline and chert samples from locality 2 (text fig. 1). 1. Slightly altered diatom frustules. Silicon distribution map of figure 1. Feathery cristobalite crystals lining vug in plant fossil (pl. 7, fig. 1). Silicon distribution map of figure 3. Diatom mold filled with cristobalite crystal blades. Silicon distribution map of figure 5. 9323'pr GEOLOGICAL SURVEY PROFESSIONAL PAPER 1111 PLATE 4 OPALINE AND CHERT SAMPLES PLATE 5 FIGURES 1—6. Scanning electron micrographs of opaline material. Figures 1—4 from locality 4 (text fig. 1). Figures 5, 6 from locality 2 (text fig. 1). 1. 2. 3. 9" Altered diatom frustule embedded in chert. Diatom frustule similar to that shown in figure 1 but with part of its outer shell gone. Well—developed cristobalite crystals line interior of mold. Diatom frustule exposed after grinding surface of opaline sample. Enlargement of cristobalite crystals in figure 2. Note scalloped and serrated edges of crystal blades. Irregular-shaped lepispheres growing in vug in chert. Symmetrical lepispheres in opaline cavity. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1111 PLATE 5 1 0 10pm L_____l OPALINE MATERIAL PLATE 6 FIGURES 1—6. Scanning electron micrographs of ostracode valves and seed etched from siliceous matrix using 10 percent hydrofluoric acid. Samples from locality 2 (text fig. 1). 1. @P‘PP’N Single ostracode valve replaced by silica. Enlargement of figure 1 showing surface features. Two ostracode valves replaced by silica. Enlargement of figure 3 showing surface features. Unidentified fossil seed replaced by calcite. Enlargement of figure 5 showing surface composed of calcite crystals. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1111 PLATE 6 6pm 1 0 10mm L__—l 5 0 10mm 6 0 Sum |_____J OSTRACODE VALVES ETCHED FROM SILICEOUS MATRIX PLATE 7 FIGURES 1—6. Scanning electron micrographs of fossil plants in the Hualapai Limestone Member. Samples from locality 1 (text fig. 1). @P‘PWF’!‘ Transverse section of fossil reed showing interior filling of chalcedony. Enlargement of reed filling in figure 1. Fossil plant molds in limestone. Enlargement of surface of fossil shown in left side of figure 3. Chalcedony filling of cavity between reed fossils in limestone. Chalcedony filling cavity between fossil plants in limestone. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1111 PLATE 7 0 100 gm 10um FOSSIL PLANTS PLATE 8 FIGURES 1—7. Algal limestone; Hualapai Limestone Member, Grapevine Canyon. Specimen 846A. 1, 2. Longitudinal and transverse View of nearly parallel branches of calcareous algae. 3, 4. Transverse view of calcareous algae branch showing radial banding. 3, White areas shown by arrow are detrital quartz fragments. Circular bands are composed of ra- diaxial calcite. Light bands are composed of fibrous calcite up to 0.2—0.3 mm long. Narrower dark bands are formed by subhedral calcite crystals 20 to 50 mm in size and possibly iron-rich inclusions. 5, 6. Longitudinal view, showing arched dark and light banding of algal branches. Figure 6 shows in detail fibrous calcite that forms bands. 7. Scanning electron micrograph of calcareous algae, Calcite rhombs range in size from 3 to 15 m. Siliceous, microcrystalline silica (S) and clay (cy) minerals stand in relief. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1111 PLATE 8 1. u 1” . ‘ . 4“ :w' ALGAL LIMESTONE PLATE 9 FIGURES 1—6. Pisolites and microstructures, Hualapai Limestone Member. 1, 3, 5. Pisolite structures formed by radiaxial fibrous calcite (cr); dark bands formed by iron oxide-rich zones. 3, Pisolitic structures formed by radiaxial fibrous calcite (or); dark area in upper half of view formed of microcrystalline chalcedony (ch). 5, Details of light bands of radiaxial calcite (or) and dark narrow bands of iron oxide-rich calcite (D). Specimen 876B. 2, 4, 6. 2, Rock is formed of volcanic fragments (vf), dark areas, silt- and sand-size grains of angular quartz, and algal nodules and branches (A). 4, 6, Enlarged View of structure illustrating wide light bands and narrow dark bands. Specimen 892D. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1111 PLATE 9 0.5mm PISOLITES AND MIC RO STRUCTURES PLATE 10 FIGURES 1—6. Fenestrate peloid packstone, Hualapai Limestone Member. 1. Pellets are from 50 to 100 pm in size, have poorly defined outlines, and are composed of calcite crystals 2—4 um in size. Subrounded quartz grains are up to 0.3 mm in size. 2. Details of peloids, showing surface of peloids covered by early-stage, first-phase calcite cement that is stubby calcite crystals with pyramidal terminations growing at right angles to grain margins (cp); voids between peloids were then filled by sparry calcite (cs). Specimen 800A. 3, 4. Peloid packstone; rock is free of quartz sand grains. 3, Outlines of peloids are poorly defined; peloids are typically 50—100 pm in diameter. 4, Peloids are composed of cal- cite crystals 3—6 pm in size. Areas between peloids are filled by sparry calcite cement (cs). Specimen 800B. 5, 6. Peloid packstone. Peloids are formed by 4—8pm calcite crystals. Outlines of individual peloids are poorly defined. Area between peloids is filled by sparry calcite (cs). Spec- imen 800C. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1111 PLATE 10 5 ~ ' 1’ ’v" FENESTRATE PELOID PACKSTONE PLATE 11 FIGURES 1—6. Algal oncolites; Hualapai Limestone Member, Grapevine Canyon. 1. Cut and polished surface of algal oncolites. 2, 3. 2, Transverse thin section of oncolites showing concentric banding. 3, Enlarged view of banding composed of fibrouscalcite, wide light bands, and narrow dark concentric bands. 4. Scanning electron micrograph of algal oncolites, cavity lined with crystals of calcite. 5, 6. Cavity within oncolite lined with microcrystalline chert. 5, Polished and etched surface of algal oncolite illustrating small (5—10 pm) calcite rhombs and silica-lined cavity within banding. 6, Scanning electron micrograph of silica-lined cavity shown in figure 5. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1111 PLATE 11 I , ‘ . , , ' ‘ ' ‘ ‘ ,~ , 1V . - . 1 ALGAL ONCOLITES, GRAPEVINE CANYON PLATE 12 FIGURES 1—6. Juncus? sp. indet. Specimen 876. 1, 2. Transverse section of stems of Juncus? sp. ‘Dark areas are calcite preservation, light areas are opal. 3-6, Enlarged views of stems, showing plant wall and cell structure; fine details of plant structure are preserved by opal and jasper. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1111 PLATE 12 OPALIZED PLANT STEMS, JUNCUS? SP. INDET. PLATE 13 FIGURES 1—6. Peloid packstones; Hualapai Limestone Member, Grapevine Canyon. 1. Peloid packstone; outlines of peloids are blurred. Peloids are 75—125 nm in size and composed of calcite crystals 5—10 pm in size. Specimen 800C. 2, 4, 6. Arenaceous peloid-pisoid-oncolitic packstone. Specimen 809. 2, Typical concentric- banded arenaceous pisoid. Quartz sand is subrounded, spaces between pisoids are composed of recrystallized micrite and contain quartz sand. 4, Enlarged View of pisoids, showing centers to be formed by peloids, and exterior by crudely banded concentric rings of micrite. Space in upper right of view is a void (V). 6, Enlarged View of area between peloids, showing space filled by sparry calcite (cs). 3. Peloid packstone, illustrating fuzzy outlines of peloids and sparry calcite (cs) filling of void between peloids. Specimen 800B. 5. Arenaceous peloid packstone. Photomicrograph shows sequence of diagenetic events. Formation of micritic peloids: deposition on peloids of first4phase calcite cement as stubby calcite crystals with pyramidal terminations growing at right angles to grain margins (cp); and final void filling by sparry calcite (cs). Specimen 800A. PROFESSIONAL PAPER 1111 PLATE 13 3 0 0.2mm 4 0 0.5mm l_.—_l 5 0 0.2mm 6 0 0.2mm L——l l——.J PELOID PACKSTONES PLATE 14 FIGURES 1—6. Hualapai Limestone Member. First canyon north of Grapevine Canyon. 1. Dolomitic peloid packstone. Calcite crystal averages 100—125 pm in size, dark relict peloids are now inclusions within larger calcite crystals. Rock is porous. Dolomite rhombs are in cavity walls. Specimen 902. 2. Scanning electron micrograph of etched calcite surface. Calcite rhombs are 5—10 pm in size‘ Specimen 903. 3. Peloid packstone. Crossed nicols. Black areas are pores. Elongate gray peloids are com- posed of 4 me—size calcite crystals. White areas are sparry calcite fillings. Specimen 904. 4, 5. 4, Arenaceous peloid-mudlump packstone. Crossed nicols. Mudlump and peloids show evidence of two periods of cementation; band of gray radiaxial cement lines each par- ticle (cr), followed by sparry calcite (cs) void-filling cement. 5, Scanning electron mi- crograph of mudlump. Gray areas are 1—4 am calcite rhombs; linear white material is clay. Specimen 908. 6. Arenaceous peloid-mudlump packstone. Crossed nicols. Cemented by sparry calcite (cs) and radiaxial calcite. PROFESSIONAL PAPER 1111' PLATE 14 SPECIMENS FROM NORTH OF GRAPEVINE CANYON PLATE 15 FIGURES 1—6. Hualapai Limestone Member. First canyon north of Grapevine Canyon. 1. Arenaceous peloid-mudlump packstone. Voids are filled by sparry calcite (cs). Specimen 910. 2, 3. 2, Fenestrate arenaceous peloid-mudlump packstone. Quartz sand grains (Q) are sub- angular to subrounded. Void between micritic particles is now filled by two genera- tions of calcite; vadose silt composed of fine crystalline calcite (vs) and later sparry (cs) void filling. 3, Micrite envelope? (me) filled by sparry calcite (cs) in microspar matrix. Micrite envelope may represent former aragonite mudlump. Specimen 911. 4, 5. 4, Vuggy arenaceous peloid-mudlump packstone to lime mudstone. Sequence of vug filling is two distinct bands of radiaxial fibrous calcite (crl, crz) and void filling by sparry calcite (cs). 5, Enlarged view of small vug, showing micrite matrix (cm), two bands of light and dark radiaxial fibrous cement (crl, crz), and final sparry calcite (cs) void filling. 6. Vuggy arenaceous peloid-mudlump-lime mudstone. Microfabric is dark micrite matrix (cm), quartz sand (Q), and sparry calcite (cs) vuggy filling. Specimen 912. PROFESSIONAL PAPER 1111 PLATE 15 SPECIMENS FROM NORTH OF GRAPEVINE CANYON PLATE 16 FIGURES 1—6. Hualapai Limestone Member. First canyon north of Grapevine Canyon. 1—3. Peloid-mudlump packstone. Photomicrograph shows particle size, peloid to pisolitic con- figuration of micritic particles, and possible vadose silt (vs); area between particles is filled by sparry calcite (cs). Specimen may represent ancient caliche. 2. Enlarged View between micritic particles shows long slender calcite (cr) crystals and final sparry calcite (cs) void filling. 3, Scanning electron micrograph of etched micritic sur- face. Quartz sand (Q) grain in center of view is surrounded by calcite crystals. Spec- imen 913. 4—6. Fenestrate arenaceous peloid—mudlump packstone to lime mudstone with sparry calcite void filling. 4y Abundant subrounded quartz sand grains (Q), sparry calcite (cs), and irregular voids (V). 5, Details of void fillings, showing early stage radiaxial fibrous (or) cement followed by sparry calcite (cs) void filling. 6, Scanning electron micro- graph showing quartz grain (Q) within micrite. Calcite crystals within micrite are only 2—4 um in size. Specimen 914. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1111 PLATE 16 0.5mm SPECIMENS FROM NORTH OF GRAPEVINE CANYON PLATE 17 FIGURES 1—6. Hualapai Limestone Member. First canyon north of Grapevine Canyon. 1—3. Arenaceous peloid-lime mudlump oncolitic packstone. 1, Left half of view shows irreg- ular banded or circular objects, which are believed to be of algal origin; but interior of circular objects is made of arenaceous compound mudlumps and peloids. Right half of view shows poorly defined peloids, quartz grains (Q), and micritic matrix. 2, En- larged view of small cavity filled by sparry calcite (cs). Micritic matrix contains quartz grains (Q). 3, Scanning electron micrograph of etched surface of micrite. Linear white bands in center of view are concentrations of clay minerals. Clays are relatively abundant between calcite rhombs. 4—6. Arenaceous lime mudstone. 4, Cross section of ostracode filled by sparry calcite and surrounded by micrite. 5, 6, Scanning electron micrographs. 5, Linear fractures or stylolites filled by clay minerals. 6, Detailed view of micrite calcite crystals in 1—4-um size range, white objects between calcite crystals are clay (cy) minerals. Spec- imen 916. PROFESSIONAL PAPER 1111 PLATE 17 2 0 0.2mm L_—___.| 5 O 10/1m 6 0 3,um I—__l SPECIMENS FROM NORTH OF GRAPEVINE CANYON PLATE 18 FIGURES 1—6. Hualapai Limestone Member. First canyon north of Grapevine Canyon. 1—3. Fenestrate arenaceous peloid-mudlump-oncolitic packstone. 1, Quartz grains (Q) are subrounded and in micritic matrix. Straight object (ls) in center of mudlump may be molluscan fragment, on which algal lamination grew. 2, Details of irregular-shaped oncolite, which has center formed by peloids and quartz sand. Algal laminations are seen on outer parts. Two stages of void filling can be seen. Radiaxial cement (or), 100— 300 um wide and a final void-filling sparry calcite (cs). 3, Details of void filling, dark micritic matrix (cm), radiaxial calcite (or), and final void-filling sparry calcite cement (cs). Long-axial fibrous cement is well displayed under polarizing microscope. Spec- imen 918. ‘ 4—6. Specimen 892C. 4, Algal oncoids and platy laths of calcite set in sparry calcite cement. 5, Algal oncoids in lower left side (a0) and platy laths in upper part of view. Sparry calcite cement (cs) and void space (V). 6, Algal oncolites (a0); note large platy lath (pl) at extreme left of view and dark central band in lath; voids are filled by sparry calcite (cs). GEOLOGICAL SURVEY 3 0 0.2mm 4 0 10mm 1 SPECIMENS FROM NORTH OF GRAPEVINE CANYON PLATE 19 FIGURES 1—6. Hualapai Limestone Member. First canyon north of Grapevine Canyon. 1. Radiaxial fibrous calcite, filling void in arenaceous pellet—mudlump-lime mudstone. Marginal portion of radiaxial fibrous mosaic exhibits preferred elongation and crys- tallographic orientation of crystals normal to micritic substrate. Numerous inclusion zones can be observed in fibrous crystals. Text figure 7 illustrates crystal sizes and inclusion patterns found in this specimen. Specimen 908. 2—4. Arenaceous peloid-algal—oncolitic packstone to grainstone. Crossed nicols. 2, Right half of view shows dark and light laminations of large algal oncolite with quartz sand inclusions (Q). 4, Arenaceous mudlump and algal oncolites. Light and dark bands of algal oncolites are seen in right third of view. Radiaxial fibrous calcite filling can be observed (cr) between mudlumps and algal oncolites. Specimen 918. 3. Radiaxial fibrous calcite (crossed nicols) that appears to have developed on a micritic envelope. Interiors of micritic envelope filled with sparry calcite (cs). Inclusion zone (iz) is more or less parallel with micrite substrate. Specimen 911. 5—6. Arenaceous pellet-mudlump packstone to grainstone. 5, Radiaxial fibrous calcite void filling. Plane light. 6, Specimen shows marginal portions of radiaxial fibrous mosaic with preferred elongation and crystallographic orientation normal to substrate. Crossed nicols. Specimen 914. PROFESSIONAL P‘APER 1111 PLATE 19 GEOLOGICAL SURVEY 0.5mm 0.5mm 0.2mm 0.2mm 0.2mm 0.2mm SPECIMENS FROM NORTH OF GRAPEVINE CANYON Petrology, Sedimentology, and Diagenesis of Hemipelagic Limestone and Tuffaceous Turbidites in the Aksitero Formation, Central Luzon, Philippines By ROBERT E. GARRISON, ERNESTO ESPIRITU, LAWRENCE j. HORAN, and LAWRENCE E. MACK GEOLOGICAL SURVEY PROFESSIONAL PAPER 1112 Prepared in cooperation with the Bureau of Mines, Republic of the Philippines, and the U .S . National Science Foundation UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON:1979 UNITED STATES DEPARTMENT OF THE INTERIOR CECIL D. ANDRUS, Secretary GEOLOGICAL SURVEY H. William Menard, Director United States. Geological Survey. Petrology, sedimentology, and diagenesis of hemipelagic limestone and tuffaceous turbidites in the Aksitero Formation, central Luzon, Philippines. (Geological Survey Professional Paper; 1 l 12) Bibliography: p. 15-16 Supt. of Docs. No.: [19.1621112 l. Limestone-Philippine Islands—-Luzon. 2. Turbidites—Philippine Islands--Luzon. 3. Geology, Stratigraphic--Eocene. 4. Geology, Stratigraphic—-Oligocene. 5. Geology-Philippine Islands-- Luzon. 1. Garrison, Robert E. 11. United States. Bureau of Mines. 111. Philippines (Republic) IV. United States. National Science Foundation. V. Title. V1. Series: United States. Geological Survey. Professional Paper; 1112. QE471.15.L5U54 1979 552’.5 79-607993 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, DC. 20402 Stock Number 024-001—03220-1 FIGURE CONTENTS Page Abstract 1 Introduction 1 Acknowledgments 1 Field description and stratigraphic relations 2 Petrology of hemipelagic limestone 2 'l'uffaceous turbidites 7 Sedimentary structures 7 Composition and texture 8 Diagenetic minerals and textures 9 Traces of hydrocarbon ll Origin and diagenetic history of the tuffs 11 Zeolites 12 Sedimentological and tectonic setting 12 Significance of hydrocarbon traces in the tuffaceous sediments ------------------------------------------ 14 References cited 15 ILLUSTRATIONS Page . Maps showing location of Aksitero River section, central Luzon 2 . Composite Cenozoic stratigraphic section for central Luzon 3 . Photograph showing outcrop of thin~bedded hemipelagic limestone and tuff, Aksitero Formation ----------------------------------- 4 . Photomicrographs of hemipelagic limestone 4 . Transmission electron micrographs of micritic matrix in hemipelagic limestone 5 . Transmission electron micrographs of nannofossils and products of diagenesis in hemipelagic limestone -------------------------- 6 . Photograph showing slabbed and polished surface of tuffaceous turbidite bed 7 . Photomicrographs of turbidite luff layers 8 . Photomicrograph of turbidite tuff layer showing volcaniclastic grains embedded in matrix of small glass shards replaced by mordenite 9 . Scanning electron micrographs of zeolites in tuffaceous turbidite 10 . Schematic diagram showing depositional setting of sediments in Aksitero Formation 13 . Sketch showing early Tertiary plate tectonic reconstruction of the northern Philippines and Marianas region -------------------- 14 Ill PETROLOGY, SEDIMENTOLOGY, AND DIAGENESIS OF HEMIPELAGIC LIMESTONE AND TUFFACEOUS TURBIDITES IN THE AKSITERO FORMATION, CENTRAL LUZON, PHILIPPINES By ROBERT E. GARRISON, ERNitsro EsriRrru,1 LAWRENCE]. HORAN, AND LAWRENCE E. MACK ABSTRACT The Aksitero Formation of central Luzon is an upper Eocene and lower Oligocene sequence of evenly bedded hemipelagic limestone with a few thin interlayers oftuffaceous turbidites. The limestone con~ sists chiefly ofplanktonic foraminifers and calcareous nannofossils. with up to 30 percent of noncarbonate components, chiefly \'(>lcatiiclasti(‘ debris. The tuff layers are graded beds composed mainly of glass shards, pumice fragments. crystals, and tine-grained volcanic rock fragments. Hydrocarbons migrated into the pores of the tuffaceous layers early during diagenesis but they were subsequently flushed out and only bitumen remains. chiefly as thin coatings on grains and within pumice vesicles Later during diagenesis. zeolites tniordenite and cli— noptilolite) and secondary calcite preferentially replaced glass shards and pumice fragments Deposition of the Aksitero Formation prob- ably occurred at depths of at least l,()()0 meters within a subsiding basin adjacent to an active island arc system. Submarine ash eruptions of silicic composition caused volcaniclastic turbidity currents that oc- casionally reached the basin floor. The more proximal facies ol'these volcaniclastic deposits may be prospective for hydrocarbons. INTRODUCTION This study is a product of a cooperative research pro- gram on the origin and distribution of energy re- sources along tectonically active plate margins in east Asia. One phase of this research has been concerned with the sedimentology of basinal sedimentary se- quences that are potential source rocks for hydrocar- bons. Among rocks of this kind we have concentrated mainly on petroliferous diatomaceous deposits of late Tertiary age in many parts of the north circum-Pacific area (Garrison, 1975). The limestone and tuffaceous turbidites of the early Tertiary Aksitero Formation described here are a somewhat different kind of ba- sinal sequence which appears, however, to have simi- lar potential for the generation of hydrocarbons. Tertiary sedimentary rocks exposed along the Ak- sitero River in the foothills of the Zambales Mountains west of the Central Valley of Luzon (fig. 1) form one lBureau ofMines, Manila, Republic ofthe Philippines. of the standard biostratigraphic reference sections for Cenozoic rocks in the Philippines. Among other work- ers, Corby and others (1951, p. 105—108), Bandy (1962, p. 18—23; 1963, p. 1738—1739), Amato (1965, p. 13—23), Divino-Santiago (1963, p. 73), Grey (1967, p. 20), Roque, Reyes, and Gonzales (1972, p. 17—19), and Ingle (1975, p. 840—848) have referred to this section. The sedimentary rocks in this sequence consist of tufi'a- ceous mudstone and sandstone of the Moriones For- mation underlain by fine-grained foraminiferal lime- stone with thin calcareous tuff interlayers (fig. 2). These basal calcareous rocks were formerly considered part of the Moriones Formation and informally known as the Bigbiga limestone (Bandy, 1962, p. 18). Amato (1965, p. 13, 14) proposed the name Aksitero Forma- tion for the limestone unit and some overlying clastic rocks. On the basis of rare benthonic foraminifers, he suggested that the limestone was deposited in a deep bathyal environment, at water depths in excess of 1,000 m (see also Bandy, 1963, p. 1738; Ingle, 1975, p. 840). Our study of the petrology and sedimentary structures in these rocks generally confirms Amato’s assessment and provides additional details on the sed- imentological and diagenetic history of these rocks. In addition, we propose a depositional model that has im- plications for the prediction of lateral facies relations and possibly also for the occurrence of hydrocarbons. ACKNOWLEDGMENTS David Bukry identified nannofossil forms in electron micrographs of the limestone. We are grateful to James Boles, Ray Christopher, R. V. Fisher, R. S. Fiske, Azuma Iijima, J. C. Maher, M. J. Terman, and A. C. Waters for discussions and suggestions. This re- port was prepared in cooperation with the Philippine Bureau of Mines, and we wish to acknowledge the help of Director Juanito C. Fernandez, Mr. Felipe U. Fran- cisco, and Mr. Oscar Crispin. The work was funded in l 2 HEMIPELAGIC LIMESTONE AND TUFFACEOUS TURBIDITES, AKSITERO FORMATION 120°00' 121°00' | I 17°oo' Y ‘Q San Fernando Q oBaguio $ 16° 00' 15° 00' 1s°oo’ \ I m ‘ o x o , _ / _, 15 30 g \\ Tarlac // \ \ \‘ A [I l ,l’ .7 /") z I/ I —1 ( ( ,’ > < - 1’ ( if! 2 l oSan Miguel U) \\ (6 / \ I \\ I ,1 «L ,’ ° 1 l 15 oo — I/ K ~ 1' I :3 | FIGURE 1.—Location of Aksitero River section, central Luzon, Phil- ippines. A, Index map of central Luzon showing location of Aksi- tero River. B, Enlarged view of area near Zambales Mountains showing location of section. Dashed line indicates approximate boundary of Central Valley. part by the National Science Foundation and carried out in conjunction with the Circum-Pacific Council Map Project. FIELD DESCRIPTION AND STRATIGRAPHIC RELATIONS The Aksitero Formation at its type area along the upper Aksitero River is about 200 m thick (fig. 2). On the basis of planktonic foraminifers, Amato (1965, p. 13—14) assigned it a late Eocene and early Oligocene age. According to Roque, Reyes, and Gonzales (1972, p. 18, 19), the lower 148 m of the formation at the type area is mainly limestone of Eocene age, and the upper 52 In is interbedded limestone and tuffaceous sand- stone of Oligocene age. The unit lies unconformably above highly weathered, poorly exposed, and appar- ently complexly deformed mafic to intermediate ig- neous rocks of the Mesozoic to early Tertiary(?) base- ment complex of the Zambales Mountains (Corby and others, 1951, p. 105; Philippines Bureau of Mines, 1976, p. 63—65). This complex may be in part an ophiol- itic sequence (Ingle, 1975, p. 848). Overlying the Ak- sitero Formation is the Moriones Formation of early and middle Miocene age composed of tuffaceous mud- stone, sandstone, and conglomerate; the contact be- tween the two units is poorly exposed, but apparently it is unconformable and locally may be faulted. The Aksitero Formation is composed principally of thin-bedded pale-yellow to yellowish-white fine—grained limestone that ranges from argillaceous and slightly fissile to nearly pure, very hard, and conchoidally frac- tured (fig. 3). A few limestone beds are light greenish gray or pink. Interlayered with the limestone are a few beds of zeolitized tuffaceous turbidites, most less than 10 cm thick. PETROLOGY OF HEMIPELAGIC LIMESTONE The predominant rock in the Aksitero Formation is fine-grained foraminiferal biomicrite with tests of planktonic foraminifers dispersed in a nannofossil-rich micritic matrix (fig. 4). Pelagic microorganisms are thus the most common constituent, but the limestone also contains up to 30 percent noncarbonate detrital components and is thus best categorized as a hemipe- lagic deposit. Both thick- and thin-shelled planktonic foraminifers are present; compactional crushing com- monly has fractured the walls of the thin-shelled forms (figs. 4C and 4D). This kind of deformation probably also produced the abundant small broken fragments of foraminiferal shell walls dispersed in the matrix. Comparatively rare benthonic foraminifers make up less than 5 percent of the foraminiferal population, and PETROLOGY OF HEMIPELAGIC LIMESTONE AGE GENERALIZED COMPOSITE SECTION ESTIMATED PLANKTONIC STRAT'GRAPHIC AKSITERO RIVER MANAOAG ANTICLINE PALEOBATHYMETRYJN FORAMINIF- THICKNESS, SECTION THOUSANDS OF METERS ERAL ZONES IN METERS O 1 2 I W N23 0.0. co 00 I — o Pleistocene [Tr—“.7; I 0 00 O O O I °. “399.“? ‘5’ | 2:”: I N22 2277: . ....... | — 500 M —/m— 333 T”? I N21 :3 : 2:. I Pliocene — M—m I . . 'o'ébi ' 5:30;; Rosario 3:”;__”; I Formation — ‘:/1:; I N19 — 1000 ' - \ FAULT o oo \ \ N17 5;." ' ’_ \ Late Miocene '7ij \ \ N16 EXPLANATION Tarlac ? 7M3— \ El Formation — ~ 31:33: \ \ N15 Shale . (WK—7" \ i— F? f” 1’"; I Claystone Malinta 7;.7: "'0' 51; I m? - Formation _ .9 if. CECE? I Mudstone Middle Miocene 0000 re ij— | :01?- A I m—l‘“ c; '06? 3? I N12 Siltstone m 3— A .— 4. r ; Moriones 1:1”,- " / N11 Sandstone Formation _ .5“ MA ' g o A W 7 / N9 —/m § ' / Early Miocene ’1. fi, .3 / N8 Conglomerate 0 O . - O O . W W, m @ Early Oligocene and Aksitero *3 * A If / Limestone Late Eocene Formation 1 I, I. ‘ . ' / P15 I A A A W Tuff Mesozoic to 32:32? /\_/“’ E early Tertiary I?) Zambales M Mountains M Deformed basement M (Includes serpentine, \/\/\/\/ basalt, and gabbro) FIGURE 2.—Composite Cenozoic stratigraphic section for central Luzon, with paleobathymetric curve based on microfauna. Modified from Ingle (1975, p. 840). in one specimen we noted very rare echinoderm frag- ments. Other microfossils present in generally small numbers are radiolarians, sponge spicules, and phos- phatic fish remains. Very fine grained cristobalite has replaced most shells of the siliceous microfossils, and secondary calcite fills some radiolarian molds. Nannofossils include spherical shells of Thoraco- sphaera, coccoliths, and discoasters (figs. 5, 6A, 60). Among the discoasters is one species, Discoaster bar- badiensis Tan Sin Hok (figs. 5A and 6A), whose range of middle to upper Eocene (Bukry, 1973, p. 659) con- firms Amato’s (1965, p. 13, 14) age dating of the lower part of the Aksitero Formation. Also present are coc- coliths of the genera Coccolithus and Reticulof'enestra (David Bukry, written commun, 1976). The chambers of the planktonic foraminifers contain a variety of different materials. Nannofossil-rich mi- crite partly or entirely fills some chambers. Others contain minerals that either precipitated in void space or replaced micrite; these minerals include secondary 4 HEMll’ELAGlC LIMESTONE AND TL’FFACEOUS TURBIDITES, AKSl'l‘liRO FORMATION FIGURE 3,—0utcr0p of thin-bedded hemipelagic limestone and tuff in Aksitero Formation, Aksitero River section. calcite (fig. 60), zeolites (probably clinoptilolite), au- thigenic smectites, optically length—fast chalcedony, and a light-brownish very fine grained mineral with very low birefringence that X-ray diffraction suggests is cristobalite. Very commonly, cristobalite partly re- places the micritic fillings of shell chambers. Appar- ently the interiors of foraminiferal chambers provide a chemical milieu that is conducive to the growth of a variety of secondary minerals. Inorganic detrital components in the limestone ap- pear to be largely of volcanic origin and are present in various amounts. Measurements of insoluble residues in nine samples indicated a range of 13 to 30 percent, with an average of 21 percent. Some of this noncar- bonate material consists of clay minerals dispersed in the micritic matrix (fig. 5C). Scattered sand- to coarse- silt-size grains include the following: FIGURE 4.—Photomicrographs of hemipelagic limestone in Aksitero Formation. Plane-polarized light. A, Planktonic foraminiferal tests and volcanic detritus in nannofossil-rich micritic matrix. Embayed plagioclase clast (F); altered clast of pumice (V). B, Planktonic for- aminiferal tests and volcanic detritus in micritic matrix. C, Abun- dant spicules and compactionally flattened foraminiferal tests. D, Lamina rich in foraminiferal tests, probably the product of winnow- ing of micrite by weak bottom current. Many thin-shelled tests are broken, whereas thicker shelled ones tend to be intact. PETROLOGY OF HEMIPELAC-IC LIMESTONE 5 FIGURE 5,—Transmission electron micrographs of micritic matrix in hemipelagic limestone, Aksitero Formation. Samples prepared by two-stage replica method (Fischer and others, 1967). A, Micrite con- taining coccoliths and discoasters, including Discoaster barbadien- sis Tan Sin Hok, at top. B, Micrite composed largely of coccoliths and fragments of coccoliths. Several coccoliths have overgrowths of secondary calcite crystals (center left). C, Micrite containing coc- coliths, authigenic calcite, and clay minerals (dark particles). Well- preserved coccolith (lower left) is CoccoIithus pelagicus (Wallich) Schiller. D, Nearly tangential section through shell of Thoraco- sphaera. Surrounding micrite consists of discoasters, coccolith frag- ments, authigenic calcite, and clay minerals. 6 HEMIPELAGIC LIMESTONE AND TUFFACEOUS TURBIDITES, AKSITERO FORMATION FIGURE 6.—Transmission electron micrographs of nannofossils and products of diagenesis in hemipelagic limestone, Aksitero Forma- tion. Samples prepared by two-stage replica method (Fischer and others, 1967). A, Discoaster barbadiensis Tan Sin Hok. This species ranges from middle to late Eocene. B, Lithoclast of tuffaceous mud— stone, largely recrystallized to secondary smectite that shows well- developed 001 cleavage faces. C, Micritic matrix has undergone re- crystallization causing partial replacement of many nannofossils. Circular Thoracosphaera shell contains relatively coarse, subhedral to euhedral crystals of secondary calcite that have replaced part of coccolith filling. D, Volcanic glass shard. Acicular crystals of mor- denite and subhedral to euhedral crystals of secondary calcite have replaced glass. TUFFACEOUS TURBIDITES 7 1. Euhedral to broken or embayed angular crystals of plagioclase (fig. 4A). 2. Elongated fragments of altered pumice with long tube vesicles (figs. 4A, 4B). Most appear to be replaced by fine-grained zeolites or coarse secondary calcite. 3. Altered volcanic glass shards, many showing round-bubble vesicles. Diverse minerals re- place the shards; these include zeolites (fig. 6D), cristobalite, smectites, chlorite, and large crystals of calcite. 4. Lithoclasts of brown mudstone composed of well-crystallized secondary smectites that have a distinctive "herringbone” pattern with two preferred crystal orientations (fig. BB). Most of these appear to be altered tuf- faceous mudstone, because in some the out— lines of glass shards are still visible along with radiolarian skeletons. Chlorite is the major component of these lithoclasts in the light-greenish—gray limestone. 5. Lithoclasts composed of brown to yellow vol- canic glass fragments replaced by smectites; some lithoclasts contain small plagioclase laths. In addition to the diagenetic effects noted above, electron microscopy shows that the micritic fraction has undergone some recrystallization. Neomorphic microspar has completely replaced some nannofossils (figs. 5, 6C). The limestone is unlaminated and has indistinct mottling probably caused by burrowing. Burrows are not conspicuous, however, because the sediment was very homogeneous, without the contrasts in color and texture that normally accentuate burrow fillings. In thin section, the common absence of strongly preferred alinement of linear fabric elements (such as spicules) parallel to bedding also suggests disruption due to bur— rowing. Some discontinuous laminae in the-limestone contain concentrations of foraminiferal tests and sand- size detrital grains. The laminae suggest sporadic win- nowing of the sediment by bottom currents (fig. 4D). TUFFACEOUS TURBIDITES Thin graded beds of tuff, which we interpret as tur- bidites, are interbedded within the sequence of pelagic limestone. These beds range in thickness from 3 to 15 cm, and in aggregate they form 5—10 percent of the total thickness of the Aksitero Formation. SEDIMENTARY STRUCTURES Most individual tuff beds consist of a basal dark, sand-size part that grades into an upper part of very fine grained, hard, zeolitized, yellowish-white rock. The upper part has a pronounced conchoidal fracture and resembles porcelanite. A few tuff layers consist entirely of fine-grained porcelaneous material; they closely resemble the interbedded pelagic limestone in outcrop. The fine-grained tuffs commonly contain sparse, very faint burrows filled with pelagic lime- stone. A few beds show complete Bouma sequences, with a to e layers (Bouma, 1962, p. 49—51) in which fine-grained porcelaneous tuff forms the d interval. One sample shows a composite of two turbidite lay- ers that form a single bed (fig. 7). The lower layer con- tains Bouma a, b, and c intervals. The c interval shows small faint ripple marks and is erosionally truncated by the overlying turbidite layer (see Yamada, 1973, p. 589). The upper turbidite layer contains less sand-size FIGURE 7.—Slabbed and polished surface of tuffaceous turbidite bed, Aksitero Formation, containing two turbidite layers. Letters a, b, and c indicate Bouma intervals. Interval a is massive and b is lam- inated; both intervals contain numerous pumice fragments (fig. SC). Dark color results from staining by bitumen. The c interval contains faint small-scale ripple marks and cross-laminations. The upper tur- bidite layer lies just above the c interval. Poorly sorted basal part (x) contains coarse-sand-size crystals and lithic clasts, and changes upward rather abruptly into very fine grained zeolitized tuff (y) that is very hard and porcelaneous. Top of porcelaneous interval has faint burrows, most noticeable at upper right. Upper turbidite layer is generally finer grained than lower. 8 HEMIPELAGIC LIMESTONE AND TUFFACEOUS TURBIDITES, AKSITERO FORMATION and more fine-grained volcanic material. The grada- tion to the upper porcelaneous part of the tuff is rel- atively abrupt, perhaps due to a sedimentological break between the upper low-density and lower high- density parts of the turbidity flow (Middleton, 1967, p. 487-490; Yamada, 1973, p. 593). COMPOSITION AND TEXTURE Four main components make up the basal sandy parts of the turbidites: crystals, altered glass shards, altered pumice fragments, and lithoclasts of devitrified volcanic glass (fig. 8). The crystals are largely plagio- clase; they range from euhedral to rounded to angular and broken; some are strongly embayed. Many show FIGURE 8.—Photomicrographs of turbidite tuff layers, Aksitero For- mation. Plane-polarized light. A, Fine-grained zeolitized tuf‘f from porcelaneous upper part of turbidite layer (fig. 7, y). Outlines of small glass shards are faintly visible. Mordenite has replaced shards. B, Basal part of tuffaceous turbidite above hemipelagic lime- stone (L). Contact is irregular and erosional; angular crystals and lithic fragments in turbidite are poorly sorted. C, Large grain in upper center is highly altered pumice fragment replaced by coarse pronounced oscillatory zoning. The glass shards typi- cally show vestiges of round—bubble vesicles. At least two types of pumice fragments are present. The most abundant type contains rather sparse round—bubble vesicles. The other contains long-tube vesicles (see Fiske, 1969). Secondary zeolites and calcite have re— placed both the shards and pumice fragments, as dis- cussed below. Fine- to medium-sand-size lithoclasts of fine-grained volcanic rock are very abundant in the basal parts of the tuffs. Felted masses of feldspar microlites compose some of the lithoclasts, but the most abundant frag- ments have an intersertal texture of feldspar micro- lites in a devitrified glass matrix. Some lithoclasts are flow banded, and others have remnants of spherulitic calcite and Clinoptilolite. Clinoptilolite also fills many vesicles. Black material is mostly bitumen, which lines most of the vesicles. Specimen from interval a of turbidite bed shown in figure 7. D, An- gular crystals and lithic fragments from same specimen as SC. Dark grains are bitumen~impregnated mudstone lithoclasts. Fractures in feldspar crystal (center) also contain bitumen. Irregular mosaic of fine-grained calcite crystals cements grains. TUFFACEOUS TURBIDITES 9 structures with contraction cracks. Although they are too small and fine grained to be identified with cer- tainty, most of the lithoclasts appear to have been de- rived from extrusive or shallow intrusive rocks of dac- itic or trachyandesitic composition. In addition to the volcanic components, most of the tuffs also contain small amounts of pelagic sediment; the pelagic sediment suggests erosion, entrainment, and redeposition of this sediment during emplacement of the turbidites. Redeposited pelagic components are present either as micrite and sparse planktonic foram- inifers or more rarely as irregularly shaped lithoclasts of pelagic limestone. Other minor components include radiolarian tests, poorly preserved bryozoans, quartz, pyroxene, and amphibole. Nearly all the sand-size grains in these rocks are very angular (figs. 8 and 9). The sandy parts of the tuffs range from poorly sorted to rather well sorted. The poorly sorted parts have abundant fine-grained matrix that commonly is a mixture of small shards and micrite. The well-sorted parts, which are most common near the base of the turbidite beds, contained appre- ciable porosity before pore-filling cementation by cal- cite and zeolites. The fine-grained porcelaneous upper parts consist chiefly of small glass shards that are replaced by the zeolite mineral mordenite (fig. 8A). Some small shards are parts of the curved walls of vesicles, but most are elongated fragments that range from about 30 to 100 micrometers long and are 5—10 micrometers thick. Sorting according to density of the grains produced reverse size grading within the a intervals of some tur- bidite beds. The lowest parts of these beds contain mainly plagioclase crystals, shards, and felted volcanic lithoclasts; these are overlain by abundant large frag- ments of pumice whose average grain size greatly ex- ceeds that below (see Fiske and Matsuda, 1964, p. 87— 88; Fiske, 1969, p. 4; Yamada, 1973, p. 590—591;Niem, 1977, p. 49—61). These pumiceous layers were very po- rous and, as described in a following section, became saturated with hydrocarbons and now stand out as dark layers (fig. 7). DIAGENETIC MINERALS AND TEXTURES Diagenesis has profoundly altered the composition and texture of the tuff layers. The most conspicuous diagenetic effects are those produced by growth of zeo- lites as replacements and as cements. Clinoptilolite and mordenite (figs. 8A, and 10) were identified by X- ray diffraction; the Clinoptilolite identification was confirmed by Alietti’s (1972) heating test. Mordenite (figs. 8A, 100, and 10B) completely replaces the small FIGURE 9.—Photomicrograph of turbidite tuff layer, Aksitero For- mation, showing angular sand-size volcaniclastic grains embedded in fine-grained matrix of small glass shards replaced by mordenite. Coarse secondary calcite has replaced most of coarse volcaniclastic grains. Large pumice fragment (P) at upper right has been re- placed by several calcite crystals. Single calcite crystals have re- placed glass shards, some showing broken edges of round-bubble vesicles (lower half). Twinned plagioclase grain (F) remained un- affected. Partly crossed nicols. glass shards in the upper, porcelaneous parts of the turbidite beds and also occurs as a cement between the fragments to form a dense and very hard rock. In the lower, sandy parts of the turbidite layers, both mor- denite and Clinoptilolite (figs. 80 and 10) replace glass shards and pumice. Clinoptilolite also forms even rims of cement crystals that line intergranular pores and vesicles within pumice fragments (fig. 10A). Mordenite, which is much more abundant than cli- noptilolite, occurs as very small needlelike length-fast fibers less than 2 micrometers in diameter and some tens of micrometers long. Clinoptilolite is present most 10 HEMIPELAGIC LIMESTONE AND TUFFACEOUS TURBIDITES, AKSITERO FORMATION FIGURE 10.—Scanning electron micrographs of zeolites in tuffaceous turbidite, Aksitero Formation. Specimen is from basal part of graded turbidite (c in fig. 7). A, Euhedral crystals of clinoptilolite that formed as intergranular cement. B, Elongate clinoptilolite crys- tals in altered pumice fragment that in part replaced pumice and in part grew into vesicle cavity, C, Rosettes of acicular mordenite crystals in pumice fragment with long-tube vesicles. Mordenite both replaced glass in pumice and filled voids. D, Zeolites filling vesicle in pumice fragment. Acicular mordenite at top; euhedral clinoptil- olite at bottom. TUFFACEOUS TURBIDITES 1 1 commonly as transparent, length-slow, elongated crys- tals up to hundreds of micrometers long, tens of mi- crometers wide, and having blunt crystal termina- tions. In replacing fragments of pumice, individual clinoptilolite crystals grew across vesicle walls so that they are in part void-filling (see Iijima, 1971, p. 334— 336). Individual mordenite fibers likewise transect the edges of the glass shards that they replace. Secondary calcite is widespread and abundant in these tuffs as intergranular cement and as replace- ments of glass shards and pumice fragments. In the sandy parts of the tuffs, it locally forms a cement of irregularly interlocking equigranular crystals be- tween volcaniclastic grains (fig. 8D). Textural rela- tions show that the calcite cement predates the zeolitic cement and presumably also the zeolitization of the shards and the pumice. Secondary calcite is most abundant, however, as a second, post-zeolite generation of coarse-grained crys- tals that replaced glass shards and, to a lesser extent, crystals of plagioclase and pyroxene. Euhedral calcite rhombs, 15—50 micrometers in maximum dimension, also locally replace parts of the intergranular clinop- tilolite and calcite cements. Commonly crystals of cal- cite up to 1 mm across preferentially replace shards or pumice fragments, although the surrounding fine- grained tuffaceous matrix remains unaffected (figs. 80 and 9). In some instances the late—generation second- ary calcite forms perfect pseudomorphs after shard and pumice fragments that had been previously replaced by zeolites (fig. 9). Outlines of replaced clinoptilolite crystals are visible within the secondary calcite in places, clearly indicating that calcitization followed the zeolitization of the volcanic fragments. Other secondary minerals include authigenic smec- tites, which fill some vesicles in pumiceous fragments, and length-slow chalcedony that occurs very locally as a replacement of glass shards. Scanning electron mi- crographs suggest the presence of small amounts of cristobalite, but cristobalite could not be detected by X-ray diffraction. TRACES OF HYDROCARBON The sandy basal parts of some turbidite tuff layers have residual patches of bitumen that indicate an ear- lier stage when the pores were completely saturated with hydrocarbons. This bitumen does not fluoresce in ultraviolet light, but it does dissolve in carbon tetra— chloride, indicating that it is “dead oil.” In thin sections, the bitumen appears as thin coat- ings on the surface of some grains, in cracks in feldspar grains, and on the rims of vesicles in pumice fragments (figs. 8C and 8D). It also completely fills the vesicles of a few pumice fragments, forming a unique type of grain composed of bitumen-filled vesicles surrounded by zeolitized glass or by large crystals of calcite that have replaced the zeolitized glass. In one specimen, bitumen-impregnated micrite fills a foraminifer shell whose pores are also filled with bitumen. Some very porous grains, perhaps originally lithoclasts of tuffa- ceous mudstone, are so thoroughly impregnated with bitumen that they are black and opaque in thin section (fig. 8D). The pores in the sandy, relatively well sorted parts of the tuffs apparently became saturated with oil early during diagenesis, because textures show that the oil predated cementation, zeolitization, and calcitization. The oil appears to have been flushed out subsequently, so that voids were reopened and later filled by calcitic and zeolitic cements. Within porous pumiceous and mudstone clasts, however, much oil remained trapped in vesicles and very small pore spaces. Consequently, layers rich in these kinds of clasts, especially pumice, tend to be dark because of residual bitumen (fig. 7). ORIGIN AND DIAGENETIC HISTORY OF THE TUFFS The sedimentary structures and the entrained pe- lagic carbonate sediment in the graded tuff layers leave little doubt that they were deposited by turbidity currents. The major question about the origin of the tuffs is whether the constituent grains formed during subaerial eruptions with subsequent reworking and redeposition by turbidity currents or during subma- rine ash eruptions of the kinds described by Fiske and Matsuda (1964, p. 93—96) and Fernandez (1969, p. 35— 36). The composition of the tuffs suggests an entirely submarine origin because constituents are absent that would indicate source areas in shallow water or on land. With the exception of a single poorly preserved bryozoan skeleton, no benthonic shallow-water fossils were found in the tuffs. There are no wood fragments or plant remains. Indications of subaerial eruptions, such as accretionary lapilli (see Fiske, 1963, p. 402), also are absent. No significant amounts of terrigenous clastic sediment are present, and well-rounded grains, indicating multicycle sedimentary clasts or earlier abrasion in a high-energy environment, are likewise absent. The limited composition and very angular grains in this tuff thus strongly suggest that these turbidites were fed from underwater eruption columns of pyro- clastic debris. The tuffs are similar to dacitic tuffs in the Miocene Tokiwa Formation of Japan that Fiske and Matsuda (1964, p. 82—105) have shown were prod- ucts of subaqueous ash flows and turbidity currents. They also somewhat resemble Quaternary lacustrine l2 HEMIPELAGIC LIMESTONE AND TUFFACEOUS TURBIDITES, AKSITERO FORMATION subaqueous pumice flow deposits in central Japan as described by Yamada (1973, p. 587—593), although some of the deposits may have been erupted on land and flowed into the water where turbidity flows were generated. The telescoping of the two turbidite layers in the bed illustrated in figure 7 resembles parts of the Tokiwa Formation where, subaqueous settling of the eruptive cloud around the vent caused repeated small surges of sediment on the sea floor. The surges in turn generated repeated turbidity currents that deposited numerous thin tuffaceous turbidites. Fiske and Mat- suda (1964) noted that sequences produced by this kind of deposition are doubly graded: grain size decreases upward over some tens of meters as well as within the individual thin beds. The bed illustrated in figure 7 is also doubly graded. Textural relations in these tuffs indicate the follow- ing sequence of postdepositional events: 1. Burial and some compaction. 2. Saturation of pore space in the sandy parts of the turbidite layers with hydrocarbons. 3. Flushing of hydrocarbons by water. Some residual oil remained attached to grains and trapped intragranular pore space. 4. In parts of the sandy layers, precipitation of intergranular calcitic cement, possibly from the pore water that flushed the hydrocarbons. 5. Zeolitization: replacement of volcanic glass in shards and in pumice fragments by mor— denite and clinoptilolite, and precipitation of these minerals as void-filling cements. 6. Replacement of some zeolitized shards and pumice fragments by large crystals of sec- ondary calcite. Significant cementation 0f the tuffs must have oc- curred before the interbedded hemipelagic limestone became lithified. Although many foraminiferal tests are compactionally crushed in the limestone, forami- niferal shells as well as delicate pumice fragments show no signs of compaction within the tuff. One spec- imen consists of mordenite tuff that contains a small vertical dike filled with pelagic limestone. This rela- tion indicates that the tuff was zeolitized, was ce- mented, and became brittle while the pelagic sediment remained plastic and unlithified. ZEOLITES The association of clinoptilolite and mordenite is common in altered tuffaceous rocks of Tertiary age (Hay, 1966, p. 65—76; Iijima and Utada, 1972, p. 73— 75; Seki, 1973, p. 669; Mumpton and Ormsby, 1976, p. 2, 3). The two minerals are most abundant as replace- ments of silicic glass or as void-filling cements in tuffs of rhyolitic to dacitic composition. This kind of alter- ation may occur either in chemically active alkalisa- line lakes, as in the western United States, or during burial diagenesis, as in the Aksitero Formation and in the Tertiary of Japan. Hay’s data (1966, p. 70—72) suggest that the assem- blage clinoptilolite—mordenite is stable at burial depths between about 300 and 3,500 In. On the basis of a compilation of occurrences in Japan, Iijima and Utada (1972, p. 74) and Iijima (1975, p. 135—138) found a similar range of burial depths where clinoptilolite and mordenite coexist. They surmise, however, that temperature rather than pressure is the principal con- trolling variable, and that the mineral pair is stable at temperatures from about 55°C to less than 90°C during the burial diagenesis of marine deposits. The presence of clinoptilolite and mordenite in tuffs of the Aksitero Formation appears to be in general ac- cord with the estimates of burial depths and temper- atures noted above. Bandy (1963, p. 1739) and Ingle (1975, p. 840) have reconstructed composite Tertiary sections for central Luzon that suggest the Aksitero Formation was buried beneath a maximum of 3.5—4 km of younger sediment (see fig. 2). At geothermal gra- dients of 15°—20°C/km, values derived from a borehole measurement in northern Mindanao (Sass and Mun- roe, 1970, p. 4393), and assuming a surface tempera- ture of 20°C, the maximum temperatures at those bur- ial depths would have been in the range 70° to 100°C. The Philippine occurrence suggests further that slight textural and compositional differences may de— termine the exact site where the two minerals form. Whereas mordenite and clinoptilolite are both present in the pumice-rich, basal parts of the tuffaceous tur- bidites, mordenite is the only zeolite in the upper, fine- grained parts of the turbidites that are composed largely of very small glass shards. Seki, Oki, Odaka, and Ozawa (1972, p. 151) described an occurrence in central Japan where laumontite is the main zeolite and mordenite is confined to thin interbeds of altered fine-grained pumiceous tuff, but the zeolitization in that instance resulted from contact metamorphism. James Boles (written commun, 1976) has suggested the possibility that mordenite in these tuffs formed first by rapid alteration of the very fine grained vol- canic glass, and that clinoptilolite formed later as an alteration of the coarser shards. SEDIMENTOLOGICAL AND TECTONIC SETTING We interpret the Aksitero Formation as a lithified basinal hemipelagic sediment composed primarily of calcareous pelagic microplankton with admixed fine- grained volcanic components (fig. 11). As noted above, SEDIMENTOLOGICAL AND TECTONIC SETTING l3 Sea level Proximal volcaniclastic facies Basinal facies (Aksitero Formation) Distal volcaniclastic facies FIGURE 11.—Depositional setting of sediments in Aksitero Formation and their postulated relation to adjacent facies. A, Explosive sub- marine eruption at left generated subaqueous eruption cloud and gravity flows; latter includes turbidity currents that moved away from vent area (Fiske and Matsuda, 1964). Some turbidity currents reached deep basinal area at right, where predominant sediment is calcareous hemipelagic ooze. B, Facies developed as result of sedimentological pattern shown in 11A. the basin floor probably lay at water depths of at least 1,000 m. Turbidity currents laden with volcaniclastic sediment flowed sporadically onto the basin floor. These resulted from submarine ash eruptions and were fed from underwater eruption columns and subaqueous pyroclastic debris that had previously accumulated upslope (see Fiske and Matsuda, 1964, p. 79—105). If our interpretation is correct, the Aksitero Formation should grade laterally first into a distal volcaniclastic facies dominated by thin—bedded turbidites and then into a proximal volcaniclastic facies containing thick- bedded gravity-flow deposits (fig. 11). Eocene volcanic activity contemporaneous with dep- osition of limestone in the Aksitero Formation is wide- spread in the Philippines and the products of this v01- canism are present as andesitic, dacitic and keratophyric flows, tuffs, and agglomerates (Philippine Bureau of Mines, 1963; Gervasio, 1973, p. 313). Most reconstruc- tions of early Tertiary tectonic plates of the northern Philippines show Luzon on an east-facing arc, lying behind and above a west-dipping subduction zone (fig. 12; Moberly, 1972, p. 43—45; Karig, 1973, p. 164; Ka- rig, 1975, p. 870—876; Hilde and others, 1976, p. 13). Thus the Eocene volcanism was probably associated with an island arc-trench complex of early Tertiary age that lay along the east edge of the Philippines. At l4 HEMIPELAGIC LIMESTONE AND TUFFACEOUS TURBIDITES, AKSITERO FORMATION o a 120 125 25°N Taiwan West Philippine 800 KILOMETE RS l—l_._._._l | | l EXPLANATION _A—A—A—A—A— ? Subduction zone — Queried where uncertain, sawteeth point in direction of dip of subduction zone FIGURE 12.—Early Tertiary plate tectonic reconstruction of the northern Philippines and Marianas region according to Karig (1973, 1975). Present—day latitude and longitude are used. about the same time, Karig suggests that the South China Basin to the west of Luzon was apparently being opened as an extension marginal basin (Karig, 1973, p. 164).2 The Aksitero Formation, therefore, appears to have accumulated behind or to the west of an island arc-sub- duction complex in the eastern Philippines and on the eastern transitional edge of a major marginal basin. It directly overlies a complex ophiolitic basement se- quence that has been interpreted as a remnant of an arc-trench system of Paleogene age (Ingle, 1975, p. 848). The Aksitero Formation may have been depos— ited on the flanks of a back-arc or interarc basin formed by extensional rifting. Karig and Glassley (1970, p. 2143—2146) have described an apparently similar set- ting of Pliocene age just west of the present Mariana Island arc. Dredging on the volcaniclastic aprons of this region recovered vesicular dacitic lava and foram— iniferal sandstone containing dacitic glass. Karig and Glassley correlate submarine eruption of the dacitic lava with Pliocene extensional rifting of the Mariana 2Karig also believes that the West Philippine Basin, to the east of Luzon, opened as an extensional basin behind the Mariana Island are system during middle Eocene to early Oligocene time (fig. 12; Karig, 1975, p. 876—878). This episode of extensional tectonics, which was contemporaneous with deposition of the Aksitero Formation, was accompanied by large-scale volcanism in the Marianas. Island arc system. Redeposition of volcaniclastic sedi- ment created extensive sediment aprons composed of turbidites. These volcanic aprons grade into calcareous pelagic sediment farther to the west (Karig and Moore, 1975, p. 236; Mitchell, 1970, p. 236—237) resembling the facies changes we envisage for the Aksitero For- mation (fig. 11). Ingle’s paleobathymetric curve for the Tertiary of Luzon, reproduced in figure 2 of this report, suggests further that deposition of the Aksitero Formation oc- curred during a period of subsidence (Ingle, 1975, p. 840). This subsidence was perhaps a consequence of interarc rifting and lateral plate motion away from a spreading center within a back-arc region. In the Mar- iana Island are system, Karig (1971, p. 342) has shown that substantial vertical subsidence accompanied large-scale horizontal movements in basins behind is- land arcs. SIGNIFICANCE OF HYDROCARBON TRACES IN THE TUFFACEOUS SEDIMENTS Several workers have postulated that scarcity of suitable reservoir rocks may have limited the accu— mulation of commercial deposits of hydrocarbons in the Philippines (Merrill, 1965, pl. 65; Mainguy, 1970, p. 103). In part, the reasoning behind this assertion is that silicic to intermediate plutonic rocks, which would yield porous quartz-rich sandstones, are relatively rare in this region. Volcaniclastic deposits such as the turbidites in the Aksitero Formation, however, may contain consider- able initial porosity in‘intergranular voids as well as in intragranular voids such as vesicles. These kinds of deposits, especially those containing appreciable amounts of volcanic glass, tend to be especially sus- ceptible to diagenetic processes that occlude pore space. Among those processes is the growth of authi- genic minerals. However, if the hydrocarbons migrate into the pore space before the onset of burial diagenesis (as was the case in the Aksitero tuffs) and are not sub- sequently flushed, then the hydrocarbons would proba— bly serve to inhibit pore-filling diagenetic reactions (Fiichtbauer, 1961). From the above considerations, we conclude that tuf- faceous sandstone like that in the Aksitero Formation is a potential oil and gas reservoir. Similar volcani- clastic rocks of Neogene age contain important hydro- carbon reserves in Japan (Ikebe and others, 1967, p. 231; Katahira and Ukai, 1976). The turbidite tuffs in the Aksitero River section are of course too thin and too few to have much reservoir capacity. However, in parts of the depositional system nearer the source vents, thicker tuffaceous sand units may occur (fig. 11). REFERENCES CITED 15 For example, in the Miocene subaqueous tuffs de- scribed by Fiske and Matsuda in Japan (1964, p. 82~ 100), tuffaceous sands are present in sequences that are hundreds of meters thick, and one single bed is more than 50 m thick (see also Yamada, 1973, p. 592— 594). This kind of proximal volcaniclastic facies could contain hydrocarbons where it interfingers with or- ganic-rich rocks of the basinal facies, such as the lime— stones of the Aksitero Formation. Amato’s work (1965, p. 6, 7) suggests that such proximal facies might occur southeast of the Aksitero River section, where he de- tected a transition across a basin margin. REFERENCES CITED Alietti, Andrea, 1972, Polymorphism and crystal-chemistry of heu- landites and clinoptilolites: Am. Mineralogist, v. 57, p. 1448— 1462. Amato, F. L., 1965, Stratigraphic paleontology in the Philippines: Philippine Geologist, V. 19, no. 1, p. 1—24. Bandy, O. L., 1962, Cenozoic planktonic foraminiferal zonation and basinal development for the Philippines: Philippine Geologist, v. 16, p. 12—34. 1963, Cenozoic planktonic foraminiferal zonation and basinal development in Philippines: Am. Assoc. Petroleum Geologists Bull., v. 47, p. 1733—1745. Bouma, A. H., 1962, Sedimentology of some flysch deposits: Amster- dam, Elsevier, 168 p. Bukry, David, 1973, Coccolith stratigraphy, Eastern Equatorial Pa- cific, Leg 16, Deep Sea Drilling Project, in Van Andel, T. H., Heath, G. 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America Bull., v. 80, p. 1—8. Fiske, R. S., and Matsuda, Tokihiko, 1964, Submarine equivalents of ash flows in the Tokiwa Formation, Japan: Am. Jour. Sci., v. 262, p. 76—106. Fi'lchtbauer, Hans, 1961, Zur Quarzneubildung in Erdollagerstatten: Erdol u. Kohle, Bd. 14, p. 169—173. Garrison, R. E., 1975, Neogene diatomaceous sedimentation in East Asia: A review with recommendations for further study: United Nations, Economic Commission for Asia and the Far East, Com- mittee for Coordination of Joint Prospecting for Mineral Re- sources in Asian Offshore Areas (CCOP), Tech. Bull., v. 9, p. 57— 69. Gervasio, F. C., 1973, Geotectonic development of the Philippines, in Coleman, P. J ., ed., The Western Pacific—Island arcs, mar- ginal seas, geochemistry: Nedlands, Univ. of Western Australia Press, p. 307—324. Grey, R. R., 1967, Time-stratigraphic correlation of Tertiary rocks in the Philippines: Philippine Geologist, v. 21, p. 1—20. Hay, R. L., 1966, Zeolites and zeolitic reactions in sedimentary rocks: Geol. Soc. America Spec. Paper 85, 129 p. Hilde, T. W. C., Uyeda, S., and Kroenke, L., 1976, Evolution of the Western Pacific and its margin: United Nations, Economic Com- mission for Asia and the Far East, Committee for Coordination of Joint Prospecting for Mineral Resources in Asian Offshore Areas (CCOP), Tech. Bull., v. 10, p. 1—19. Iijima, Azuma, 1971, Composition and origin of clinoptilolite in the Nakanosawa Tuff of Rumoi, Hokkaido: Advances in Chemistry Series, Washington, Am. Chem. Soc., No. 101, “Molecular Sieve Zeolites—I,” p. 334—341. 1975, Effect of pore water to clinoptilolite-analcime-albite re- action series: Jour. Faculty Science, Univ. of Tokyo, Sec. II, v. 19, no. 2, p. 133—147. Iijima, Azuma, and Utada, M., 1972, A critical review on the occur- rence of zeolites in sedimentary rocks in Japan: Japanese Jour. Geol. and Geography, v. 42, p. 61—84. Ikebe, Y., Ishiwada, Y., and Kawai, K., 1967, Petroleum geology of Japan: United Nations, Economic Commission for Asia and the Far East, Mineral Resources Devel. Ser., v. 26, no. 1, p. 225— 234. Ingle, J. 0., Jr., 1975, Summary of late Paleogene-Neogene insular stratigraphy, paleobathymetry, and correlations, Philippine Sea and Sea of Japan region, in Karig, D. E., Ingle, J. 0., Jr., and others, Initial reports of the Deep Sea Drilling Project: Wash- ington, US. Govt. Printing Office, v. 31, p. 837—855. Karig, D. E., 1971, Structural history of the Mariana island arc sys- tem: Geol. Soc. America Bull., v. 82, p. 323—344. 1973, Plate convergence between the Philippines and the Ryu- kyu Islands: Marine Geology, v. 14, p. 153—168. 1975, Basin genesis in the Philippine Sea, in Karig, D. E., Ingle, J. C., Jr., and others, Initial reports of the Deep Sea Drill- ing Project, Volume 31: Washington, US. Govt. Printing Office, p. 857—879. Karig, D. E., and Glassley, W. E., 1970, Dacite and related sediment from the West Mariana Ridge, Philippine Sea: Geol. Soc. Amer- ica Bull., v. 81, p. 2143—2146. Karig, D. E., and Moore, G. F., 1975, Tectonically controlled sedi- mentation in marginal basins: Earth and Planetary Sci. Letters, v. 26, p. 233—238. Katahira, T., and Ukai, M., 1976, Petroleum fields of Japan with volcanic-rock reservoirs—summary, in Halbouty, M. T., Maher, J. C., and Lian, H. M. (eds), Circum-Pacific Energy and Mineral Resources: Amer. Assoc. Petroleum Geologists Mem. 25, p. 276— 279. Mainguy, M., 1970, Regional geology and petroleum prospects of the marine shelves of eastern Asia: United Nations, Economic Com- mission for Asia and the Far East, Committee for Coordination of Joint Prospecting for Mineral Resources in Asian Offshore Areas (CCOP), Tech. Bull., v. 3, p. 91-107. Merrill, W. R., 1965, Oil exploration—Philippines: Philippine Ge- ologist, v. 19, no. 3, p. 65—85. Middleton, G. V., 1967, Experiments on density and turbidity cur- rents, III, deposition of sediment: Canadian Jour. Earth Sci., v. 4, p. 475—505. Mitchell, A. H., 1970, Facies of an early Miocene volcanic arc, Ma- lekula Island, New Hebrides: Sedimentology, v. 14, p. 201—243. Moberly, Ralph, 1972, Origin and history of lithosphere behind is- land arcs, with reference to the western Pacific: Geol. Soc. America Mem. 132, p. 35—55. [6 Mumpton, F. A., and Omsby, W. C., 1976, Morphology of zeolites in sedimentary rocks by scanning electron microscopy: Clays and Clay Minerals, v. 24, p. 1—23. Niem, A. 11., 1977, Mississippian pyroclastic flow and ash-fall deposits in the deep-marine Ouachita flysch basin, Oklahoma and Arkan- sas: Geol. Soc. America Bull., v. 88, p. 49—61. Philippine Bureau of Mines, 1963, Geological Map of the Philippines, Sheet ND—51, City of Manila: Philippine Bur. Mines, scale l:1,000,000. 1976, A review of oil exploration and stratigraphy of sedi— mentary basins of the Philippines: United Nations, Economic Commission for Asia and the Far East, Committee for Coordi- nation of Joint Prospecting in Asian Ofl'shore Areas (CCOP), Tech. Bull., v. 10, p. 55—99. Roque, V. P., Jr., Reyes, B. P., and Gonzales, B. A., 1972, Report on the comparative stratigraphy of the east and west sides of the HEMIPELAGIC LIMESTONE AND TUFFACEOUS TURBIDITES, AKSITERO FORMATION mid-Luzon Central Valley, Philippines: Mineral Eng. Magazine, Philippine Soc. Min., Metall. and Geol. Engineers, v. 24, p. 11—62. Sass, J. H., and Munroe, R. J ., 1970, Heat flow from deep boreholes on two island arm: Jour. Geophys. Research, v. 75, p. 4387—4395. Seki, Yotaro, 1973, Ionic substitution and stability of mordenite: Geol. Soc. Japan Jour., v. 29, p. 669—676. Seki, Yotaro, Oki, Yasue, Odaka, Shigeo, and Ozawa, Kiyoshi, 1972, Stability of mordenite in zeolite facies metamorphism of the Oyama—Isehara district, East Tanzawa Mountains, central J a- pan: Geol. Soc. Japan Jour., v. 78, p. 145—160. Yamada, Eizo, 1973, Subaqueous pumice flow deposits in the Onikobe Caldera, Miyagi Prefecture, Japan: Geol. Soc. Japan Jour., v. 79, p. 585—597. Metamorphic Mineral Assemblages of Slightly Calcic Pelitic Rocks in and around the Taconic Allochthon, Southwestern Massachusetts and Adjacent Connecticut and New York By E-AN ZEN GEOLOGICAL SURVEY PROFESSIONAL PAPER 1113 A study of progressive regional metamorphism of pelz'tz'c schz'sts from the Taconz'c allochthon of southwestern Massachusetts and its bearing on the geologic history of the area l'NITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1981 UNITED STATES DEPARTMENT OF THE INTERIOR JAMES G. WATT, Secretary GEOLOGICAL SURVEY Doyle G. Frederick, Acting Director Library of Congress Cataloging in Publication Data Zen, E-an, 1928— Metamorphic mineral assemblages of slightly calcic pelitic rocks in and around the Tacionic allochthon, south- western Massachusetts and adjacent Connecticut and New York. (Geological Survey professional paper ; 1113) Bibliography: p. 1. Rocks, Metamorphic. 2. Petrology—Tacoma Mountains. I. Title. II. Series: United States. Geological Sur- vey. Professional paper ; 1113. QE475.Z46 515224 78—32050 For sale by the Superintendent of Documents, US. Government Printing Office Washington, DC. 20402 CONTENTS Page Page Abstract _________________________________________ 1 Mineralogy and mineral assemblages—Continued Introduction ______________________________________ 2 Model mineral compositions—Continued Acknowledgments _________________________________ 4 Chloritoid ________________________________ 22 Summary of regional geology ______________________ 4 Garnet ___________________________________ 22 Stratigraphy _________________________________ 4 Chlorite _________________________________ 23 Structure ________-_________.._- ________________ 5 Biotite ___________________________________ 23 Mineralogy and mineral assemblages _______________ 6 Muscovite ________________________________ 23 Muscovite ____________________________________ 7 Epidote __________________________________ 23 Paragonite ___________________________________ 9 Homblende _______________________________ 23 Biotite ______________________________________ 9 Plagioclase _______________________________ 23 Chlorite _____________________________________ 10 Kyanite __________________________________ 23 Chloritoid ____________________________________ 11 A model multisystem ______________________________ 23 Staurolite ____________________________________ 12 Problematic phase relations _______________________ 26 Garnet ______________________________________ 13 First appearance of chloritoid _________________ 26 Kyanite _____________________________________ 15 The chemography of garnet and the biotite-chlori- Amphiboles __________________________________ 15 toid-garnet—chlorite relations _________________ 27 Tremolite ________________________________ 15 Chloritoid-staurolite relations __________________ 30 Cummingtonite ___________________________ 15 Kyanite-bearing assemblage ___________________ 33 Hornblende ______________________________ 15 Phase relations involving epidote _______________ 34 Epidote _____________________________________ 16 Relation between epidote-bearing and hornblende- Plagioclase __________________________________ 17 bearing assemblages ________________________ 36 Ilmenite _____________________________________ 21 Cummingtonite-bearing assemblages ____________ 37 Magnetite ____________________________________ 21 Conditions of metamorphism _______________________ 38 Tourmaline __________________________________ 21 A retrograde isograd? ____________________________ 42 Quartz _______________________________________ 21 Geologic implications of overburden pressure during Model mineral compositions ____________________ 22 metamorphism __________________________________ 42 Staurolite ________________________________ 22 References cited __________________________________ 42 ILLUSTRATIONS Page FIGURE 1. Generalized geologic map of the study area ___________________________________________________ 2 2, 3. Triangular diagrams of: 2. Al"‘—Al"’—(Fetom+Mg) compositions of muscovite analyzed by use of the microprobe ____ 8 3. Atomic A(Al-alkalis)FM for biotite samples analyzed by use of the microprobe __________ 10 4 Photomicrograph of sample 376—1, showing ilmenite sandwiched between chlorite plates ___________ 11 5. Graphs showing composition profiles across garnet crystals in samples 356—1 and 487—2—4 __________ 14 6. Photomicrographs of cummingtonite assemblages for samples 170—1 and 487—2 _________________ 16 7 Graphs showing composition profiles across a hornblende crystal in sample 102—1 _________________ 17 8, 9 Photomicrographs of: 8. Coexisting staurolite and epidote in samples 356—1 and 590—1 __________________________ 17 9. Triply zoned plag'ioclase from samples 3—3 and 360—1 ____________________________________ 18 10. Graphs showing composition profiles across zoned plagioclase of samples 3—3, 360—1, and 14—1 ______ 19 11. Photomicrographs showing extreme habit adaptation of metamorphic plagioclase to the foliation sur- face of the rock, sample 423—1, and late plagioclase that includes folded foliation defined by dust trains, sample 340—1 _____________________________________________________________________ 20 12. Graph showing manganese content of coexisting garnet and ilmenite (as inclusions in garnet) in sample 356—1 ___________________________________________________________________________ 21 13. Photomicrographs of zoned tourmaline, sample 509—1, and of a mantle of tourmaline, sample 102—2-- 22 III IV FIGURES 14, 15. 16. 17. 18. 19. TABLE 1. 5° CONTENTS Diagrams showing: 14. A closed net for the quinary multisystem A1203—Fe0-MgO-CaO-K20, for the phases epidote— garnet-plagioclase-chlorite-chloritoid-staurolite-biotite—muscovite in the presence of quartz and having H20 and 02 as boundary-value components ______________________________ 15. A part of the multisystematic net of figure 14 containing the four invariant points (St), (Ep), (Bt), and (Cd) ____________________________________________________________ Photomicrographs of chloritoid—biotite assemblages _____________________________________________ ACFM diagram wherein paragonite is projected to the A apex, for phases in coexistence with epidote Graph showing experimental pressure—temperature curves pertinent to the mineral assemblages of the study area __________________________________________________________________________ Photomicrograph of sample 191—1, showing chlorite rimming magnetite __________________________ TABLES [Tables follow References Cited] Mineral assemblages in samples of rocks from within and around the Taconic allochthon, south- western Massachusetts and adjacent parts of Connecticut and New York _____________________ Localities from which samples were obtained for mineral-assemblage information __________________ Microprobe data on various minerals in samples of rocks from within and around the Taconic allochthon: Muscovite ___________________________________________________________________________ Biotite _____________________________________________________________________________ Chlorite _____________________________________________________________________________ Chloritoid ________________________________ n ___________________________________________ Staurolite __________________________________________________________________________ Garnet ______________________________________________________________________________ Kyanite ____________________________________________________________________________ Hornblende and cummingtonitie _______________________________________________________ Epidote ______________________________________________________________________________ Plagioclase __________________________________________________________________________ Ilmenite ____________________________________________________________________________ Magnetite ___________________________________________________________________________ Conventional wet-chemical analyses and the number of atoms calculated on the basis of the anhydrous formulas for selected minerals ____________________________________________________________ Comparison of wet-chemical and microprobe analyses and of the calculated chemical formulas of staurolite from sample 355—1 ____________________________________________________________ Molar volumes of minerals used to calculate slopes of the univariant curves shown in figure 15 ____ Compositions of coexisting chlorite, biotite, chloritoid, and garnet in the presence of musc-ovite, plagioclase, and quartz __________________________________________________________________ Partial mineral-assemblage data for samples of epidote—bearing assemblages _____________________ Compositions of coexisting minerals in three hornblende assemblages ____________________________ Estimates of minimum temperature of metamorphism based on the K/ (K+Na) ratios of muscovite compositions and the 2.07-kbar isolvus of Eugster and others (1972) and microprobe data ______ ?R$~¥935§9FF Page 24 26 28 35 39 42 Page 49 54 57 67 75 81 88 95 106 107 109 110 117 123 124 126 12-6 127 127 128 128 METAMORPHIC MINERAL ASSEMBLAGES OF SLIGHTLY CALCIC PELITIC ROCKS IN AND AROUND THE TACONIC ALLOCHTHON, SOUTHWESTERN MASSACHUSETTS AND ADJACENT CONNECTICUT AND NEW YORK By E-AN ZEN ABSTRACT The mineral assemblages from metamorphosed slightly calcic pelitic rocks of the Taconic Range in southwestern Massachusetts and adjacent areas of Connecticut and New York were studied petrographically and chemically. These rocks vary in metamorphic grade from those below the chloritoid zone through the chloritoid and garnet zones into the kyanite-staurolite zone. Microprobe data on the ferro- magnesian minerals show that the sequence of increasing Fe/ (Fe+Mg) value is, from the lowest, chlorite, biotite, horn- blende, Chloritoid, staurolite, garnet. Hornblende, epidote, garnet, and plagioclase are the most common minerals that carry significant calcium. Biotite is persistently deficient in alkali but is abnormally rich in octahedral aluminum to such an extent that the overall charge balance can be ascribed to an Al:K+ (Fe,Mg) diadochy. Muscovite contains small though persistent amounts of iron and magnesium in octa- hedral positions but has a variable K/ Na ratio, which is po- tentially useful as a geothermometer. One low-grade musco- vite is highly phengitic, but the white micas in rocks from metamorphic grades higher than Chloritoid zone do not con- tain significant phengite components. Chlorite is persistently high in aluminum and so its ratio of divalent ions to alumi- num is approximately that of garnet. Many garnets show pronounced zoning in manganese and less pronounced zoning in calcium. Garnet coexisting with hornblende contains a high proportion of the grossularitic component. The calcium content is significant in all the analyzed garnets, except those from a cummingtonite-bearing sample that is free of muscovite. This suggests that in slightly calcic pelitic rocks, calcium-free garnet cannot coexist with muscovite. Most of the mineral assemblages formed in the presence of excess quartz and muscovite. The phase—petrologic analysis, made with the aid of an eight-phase multisystematic model, shows the following major points: 1. Chloritoid and staurolite coexist in a definite interval of prograde metamorphism. 2. Biotite-chloritoid does not constitute an alternative as- semblage to garnet-chlorite-muscovite, because the former combination is found predominantly in the presence of the latter combination. Because the garnet contains lime, all five phases are stable together in low- Iime pelitic rocks. 3. The first appearance of staurolite in the area does not correspond to the reaction leading to the first intrinsic stable existence of this phase. Inasmuch as the first ap- pearance of staurolite is always in chlorite-bearing as- semblages, I suggest that the mapped staurolite zone marker corresponds to a reaction whereby staurolite- chlorite becomes stable. The probable lower grade chemi- cal equivalent, for example, chloritoid-aluminum sili- cate, however, has not been found in the area of study. Several staurolite-forming reactions discussed in the literature are ruled out because of the relative sidero- phility of the minerals. A second staurolite isograd involves the reaction, chloritoid+chlorite+muscovite: staurolite+biotite. A third isograd involves staurolite+ ch10rite:biotite+kyanite; this reaction is postulated on the basis of the observed assemblage biotite—kyanite- staurolite-garnet—muscovite-plagioclase—quartz—ilmenite. 4. In low-grade rocks, epidote is stable considerably before the first appearance of Chloritoid. The nature of the high-aluminum phase in low-grade rocks that leads to the formation of chloritoid remains obscure. The epidote is always rich in ferric iron (pistacite content of about 1/4 to 1/ 3). Garnet-bearing assemblages (with or with- out epidote) are formed next as metamorphic grade in- creases. The next more calcium-rich silicate is horn- blende, and despite the meager data on assemblages that include hornblende, the first intrinsic appearance of this phase has probably been recorded. At high- staurolite grade, the most calcium-rich assemblage in pelitic rocks is hornblende—garnet—biotite-plagioclase (bytownite)-chlofite-quartz; except for the details of compositions of the phases, this assemblage is not a bad approximation of an amphibolite in equivalent meta- morphic grade. Thus, the mineral assemblages from the area seem to bridge the gap between typical pelitic schists and impure amphibolites. Comparison of the mineral assemblages with hydrothermal phase-equilibrium data suggests that the approximate range of the temperature of metamorphism was 400°—600° C at a pressure not less than that of the triple point of the alumi- num silicates, or probably about 4 kbars. The metamorphism was an Acwadian event. The pressures and temperatures indi- cate that major differential uplift, on the order of about 15 km, between parts of the study area and the region of the Hudson Valley must have taken place since Acadian metamor- phism. The nature and date of this uplift are unknown. 1 2 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. INTRODUCTION The Taconic Range in the tris-tate area of Massa- chusetts, Connecticut, and New York is underlain by pelitic rocks that belong to the Walloomsac For- mation (Middle and Upper? Ordovician), the Egre- mont Phyllite (Middle and Upper? Ordovician), and the allochthonous Everett Formation (upper Pre- cambrian? and (or) Lower Cambrian?) . That these rocks have undergone regional metamorphism has long been recognized (Hobbs, 1893a, b, and refer— ences therein). Agar (1932, 1933) described some of the metamorphic rocks. His work, however, was largely of a reconnaissance type and does not contain information on the mineral assemblages or the chem- ical petrology; it was also not in the context of the recent interpretation of the structure and tectonic history of the area. Barth (1936) described the metamorphic rocks of a similar stratigraphic se- quence in the Dutchess County area, New York, farther to the south; his work has been revised and updated by Bence and Vocke (1974) and Vidale (1974a, b). As part of an effort to study the geologic history of the Taconic allochthon, N. M. R'atcliffe and I mapped the bedrock geology of the Bashbish Falls and Egremont 71/2-minute Quadrangles in the tri- state area (Zen and Hartshorn, 1966; Zen and Rat- clifl’e, 1971). In addition, the northwestern half of the Sharon Quadrangle and strips of areas adjacent to these quadrangles were also mapped (fig. 1). Many samples of these pelitic rocks, as well as samples of the carbonate—rich rocks of the Stock- bridge Formation, were studied by use of the micro- scope, X-ray diffractometer, and electron microprobe and by conventional wet-chemical analyses of min- eral separates. This paper summarizes the results of this study. The petrographic evidence suggests two episodes of regional metamorphism. The earlier episode was low grade and has been associated previously with the Taconian orogeny (Zen, 1969a, b; 1972a; Rat- cliffe, 1972). The metamorphic grade of rocks result- ing from the second episode ranges from below the biotite zone to the staurolite zone; because the micas produced by this episode have yielded K-Ar single- mineral ages of about 360 my (million years) (Zen and Hartshorn, 1966), which age is corroborated by an argon total-fusion age (M. A. Lanphere and E-an Zen, unpub. data, 1967) , this second episode is taken to be Acadian. The mineral assemblages described in this paper are almost exclusively effects of this sec- ond metamorphic episode. EXPIANATION Walloomsac Formation (Ordovician) EOWWQ) Egremont Phyllite correlated with the Walloomsac Forma- tion (Ordovician) .. OCSQEE Stockbridge Formation (Cambrian and Ordovician) Qépficd Cheshire Quartzite and Dalton Formation, undivided \ \ (Precambrian(?) and Cambrian) Cpeev Everett Formation (allochthonous) (Precambrian(?) and (or) Cambrian) — Contact—Approximately located — —— Metamorphic isograd—Approximately located Sample locations: 0274—1 Pseudounivariant assemblage chlorite+musco- vite+chlotitoid=biotite+staurolite formed in the presence of quartz by the reaction (Ep, Pg ) of the multisystem net (fig. 14) A3694 Assemblage on the low-grade side of the above reaction D5044 Assemblage on the high-grade side of the above reac- tion 42° Kingston? 4 INDEX MAP SHOWING LOCATION OF STUDY AREA FIGURE 1.-——-Generalized geologic map of the study area, showing major formations, contacts, and isograds, as well as sample control for the isograd defined by the reaction chloritoid+chlorite+muscovite:sta.urolite+biotite. INTRODUCTION 15’ 73° 73°30' STOCKBRIDGE SOUTH CANAAN STATE LINE 42° 15' 42° 00. INDEX OF QUADRANGLES 73°22'30" 73°30’ 6 KILOMETEHS 5 FIGURE 1.—Contimued. 4 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. The slightly calcic pelitic schists described here are chemically and mineralogically complex. Because these rocks are not particularly aluminum rich, the aluminum-silicate minerals, even paragonite, are rare. The rocks do contain enough calcium, however, that this component cannot be ignored in phase anal- ysis. The iron, magnesium, and potassium contents are normal, and sulfides are rare and can be ignored for the present purpose. Because of the complexity of the rocks, their study needed a guiding framework, or model; devising a model implies some simplifications of the perceived relations. In published studies of pelitic schists, other workers have made one of several possible simplifying assumptions. They have assumed the sys- tem to be chemically simple, or they have assumed the system to be mineralogically simple by limiting the number or the permissible variations of the com- positions of the minerals. To be sure, one could study the detailed chemistry of all the minerals in actual contact in each rock and derive precise information on the changes of mineral chemistry as the bulk com- position and metamorphic grades vary; however, such a laborious effort must be a second stage to a “detailed reconnaissance” that this study is intended to be. In this study, I adopted the expedient of using a model multisystem that contains all the important minerals observed and that includes all the major chemical components for these minerals. The multi- system, which is a model petrogenetic grid, allows study of the reaction relationships and prediction of discontinuous changes of phase assemblages that are used as isograds. The price one pays for this model is to use simple mineral compositions—not end- member compositions but rather fixed compositions whose atomic proportions are “averages” of the microprobe and wet-chemical analyses of minerals reported here. This expedient provides a framework whereby the myriad observations successfully find their niches, but it sacrifices prospects of under- standing the details of the variation in composition of coexisting minerals as a function of metamorphic grade. The data are not lost; they are in the tables of chemical analyses and mineral assemblages and are discussed in the text for specific reactions. They are, however, not made the center of attention of the report. I hope this reconnoitering effort will lead others to study the details in the not-too—distant future. ACKNOWLEDGMENTS Many people have contributed to this study. My thanks go first to A. E. Bence, who introduced me to the use of the electron microprobe and was responsi- ble for the collection of a substantial part of the data. Another who helped materially with the micro- probe work was L. B. Wiggins. I must acknowledge Robert Meyrowitz for the especially painstaking and superb analyses of a garnet and a staurolite. For careful mineral separations used for the chemical analyses I am deeply indebted to D. E. Lee and Cristina S. Zen. Discussions of the petrologic prob- lems with J. B. Thompson, Jr., A. L. Albee, N. M. Ratcliffe, and D. R. Wones have been most fruitful. S. W. Bailey helped to identify the polytypes of analyzed chlorite; Malcolm Ross helped to identify the polytypes of mica, and he and J. S. Huebner gave invaluable advice and help in X-ray cell measure- ments and in matters mineralogical in general. The contribution during a period of several years of Jane G. Hammarstrvom, who performed many of the tedious laboratory chores, undertook the horrendous task of microprobe-data reduction in preauto-mation days and made additional microprobe analyses, can- not be adequately acknowledged; both she and J. L. Haas, Jr. also helped with the computer software. To all these kind people and many others not specifically mentioned, I offer my gratitude. The manuscript was reviewed by D. S. Harwood, B. A. Morgan, P. H. Osberg, and D. B. Stewart, and I am truly grateful to them for their Herculean efforts to make this re- port better. SUMMARY OF REGIONAL GEOLOGY The regional geology of the area is briefly sum- marized; for more details, the reader is referred to Zen and Hartshorn (1966) and Zen and Ratcliffe (1971). STRATIGRAPHY The Precambrian(?) and Paleozoic section con- tains a basal arkose, the Dalton Formation, suc- ceeded by a vitreous quartzite, the Cheshire Quartz- ite. This is followed by a thick succession of shallow-water carbonate rocks, the Stockbridge Formation. Although heterogeneous in detail and subdivided into six members (A to F, from old to young), the Stockbridge is predominantly dolomitic in the lower part and calcitic in the upper part. Quartz-rich nodules in the units could be metamor- phosed chert nodules, as equivalent rocks at low metamorphic grades contain chert beds or nodules. SUMMARY OF REGIONAL GEOLOGY 5 At sufficiently high metamorphic grades, such nodu- lar quartz reacted with the carbonate minerals to produce calc-silicate minerals (see also Burger, 1975). Above the Stockbridge Formation and an ero- sional gap representing a Middle Ordovician regional unconformity (Zen, 1967), the Walloomsac Forma- tion is present. It is a series of gray to black, locally sulfide-rich, micaceous to calcareous pelites. Fine silty sandstone beds and silty limestone beds are minor components of the formation. The base of the Walloomsac Formation is commonly a blue-gray limestone rich in silt and mica; more rarely, this zone is a ferruginous calcareous siltstone, presuma- bly derived from iron-rich residual soil, which was extracted for iron in the 19th century through much of the area (Chute, 1945). The Egremont Phyllite, a dark gray to black pelite, is exposed in a large but totally enclosed area. For the purpose of this report, this unit is correlated with the Walloomsac Formation. The geometrically highest unit is the Everett For- mation, consisting at low metamorphic grades of green phyllite, quartzose phyllrite, and minor inter- bedded quartzite, silty limestone, and graywacke. Beds that might have been impure volcanic ash (now hornblende bearing) and beds composed of a peculiar quartzose rock of uncertain original nature that carry the assemblage cummingtonite-magnetite- garnet-biotite-quartz are present in a very few places in the formation. Purplish low-grade phyllite is evident in the northwest part of the Egremont Quadrangle and along the Mount Washington road east of Mt. Fray. One area of massive graywacke mapped near Bashb-ish Falls east of Copake pro- vided the best examples of the epidote—bearing as- semblages below the garnet zone; this graywacke is correlated by lithology with the Rensselaer Gray- wacke in the area farther to the northwest in New York State. Most parts of the Everett Formation at higher metamorphic grades are silvery green. Likewise, the parts of the Walloomsac Formation that have under- gone higher grade metamorphism have become sil- very gray, instead of dark gray to black. In both instances, the color is imparted principally by grains of white mica and feldspar that became coarser. Therefore, at higher metamorphic grades (high- almandine to staurolite zones) these two formations are locally difl‘icult to distinguish in hand specimens. Geologic context may provide a clue for mapping; the pelites of the Walloomsac also tend to be more calcareous. As seen under the microscope, the cen- ters of crystals from the Walloomsac Formation often retain dusty inclusion trains of carbonaceous material, which are relicts from lower metamorphic grades; these trains help to identify the formation. The Everett Formation is allochthonous and is a slice of the composite Tacovnic allochthon (Zen, 1967). The slice is geometrically the highest and tectonically the latest of the allochthon. In the north- west corner of the Egremont Quadrangle, purple and green slate that may belong to an earlier, tec- tonically lower slice is found geometrically beneath most of the Everett and is separated from it by the Walloomsac and by slivers of the Stockbridge. Because the Everett Formation is allochthonous, one can ask whether its metamorphism might have preceded its tectonic emplacement. The following list of observations suggests that this is unlikely: First, there is no evidence of any discontinuity in meta- morphic grade across the zone of tectonic movement. Second, the phyllites of the Everett Formation in the northwest corner of the Egremont Quadrangle are the product of the early metamorphic event. Here the early formed foliation in the allochthonous rock can be physically traced into the foliations in the carbonate sliver and in the underlying Walloomsac Formation; thus, the foliation and associated meta- morphism are interpreted to be no older than the tectonic emplacement (Zen, 1969a, stop A6). I con- clude, therefore, that the mineral assemblages of the early metamorphism of the allochthonous rocks can be interpreted to be continuous parts of the entire rock succession; thus, the mineral assemblages of the later Acadian metamorphism must also be con- tinuous through the entire rock succession. STRUCTURE The area has been deformed several times. The earliest deformation that is readily recognized is a group of isoclinal recumbent folds in the Stockbridge Formation. The age of these folds is interpreted as Early to Middle Ordovician. These large-scale pas- sive-flow folds in the carbonate units apparently produced no recognizable metamorphic effect. The second episode of folding was during the em- placement of the allochthonous rocks, which could be a late Taconian event, especially if the upper slices of the allochthon were propelled by the nappes form- ing in the crystalline basement rocks of the Berk- shire Highlands; the thrusting of the crystalline nappes is tentatively dated as Taconian on the basis of the isotope age determination of a syntecto-nic in- trusive rock (Harwood, 1972; Zen, 1976). This epi- sode of folding is interpreted to be about the same 6 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. age as the first episode of low-grade metamorphism. Ages determined by a variety of isotopic methods on rocks from Vermont (Harper, 1968; Lanphere and Albee, 1974; M. A. Lanphere and E-an Zen, unpub. data, 1967) and from southeastern New York (Long, 1962; Bence and Rajamani, 1972; N. M. Ratcliffe, oral commun., 1975) provide evidence for a late Taconian (Late Ordovician to Early Silurian) re- gional thermal and likely metamorphic event; this event is entirely consistent With the local geologic evidence for the age of emplacement of the alloch- thonous rocks. This second episode of folding is recorded as overturned to recumbent folds in all the rocks (in the Everett Formation, there are local in- dications of an earlier foliation that is folded by this second axial surface, but the evidence is meager, and the event remains to be verified). The last episode of flexural deformation is re- corded as relatively small scale, open, upright to lo- cally overturned folds, although some recumbent folding is not excluded. The foliation and the axial surfaces of the second folding were both folded by this late event, although single outcrops rarely show folding of both. This late event is associated with the Acadian orogenic and metamorphic events. These events produced the most significant metamor— phic assemblages and all the almandine—grade and higher grade assemblages in the area. Isotopic dates (Zen and Hartshorn, 1966) by K-Ar method on mus- covite (390 my.) and by Rb-Sr method on biotite (355 m.y.) from both allochthonous and autochtho- nous rocks gave Acadian ages in the southern part of the area. As shown in the section Geologic Implications of Overburden Pressure During M-etamorphism, the post-Acadian uplift may be on the order of 104 meters in the area but may amount to only about 103 m barely 3x104 m to the west. Thus, large- scale differential movement, either distributive or local, may have taken place. MINERALOGY AND MINERAL ASSEMBLAGES The mineral-assemblage data given in table 1 were obtain-ed principally by means of the petrographic microscope and were supplemented by data from X-ray diffraction of the samples. Table 2 gives the localities of samples by longitude and latitude, states the metamorphic zone and the formation, and indi- cates whether data from microprobe and (or) wet chemical analyses supplement petrographic micro- scope data for minerals of a given assemblage. Many compositions of coexisting minerals were determined by the electron microprobe. These re- sults, given in table 3, are probably good to :1 per- cent for major components and to :5 percent for minor components, though the results are reported to more significant figures. A few minerals were also analyzed by conventional wet-chemical methods, and the results are given in table 4. A few words of explanation for the acceptability of the microprobe analyses recorded in table 3 are in order. For hydrous minerals, the analyses have been recalculated by adding enough H20 to make up the fully hydrated formula: Weight percent H20 = 18.016 ( (Ohf — Oat) /Oaf) S, where OM is the number of gram atoms of oxygen in the hydrous formula for the mineral, 0,, is the num- ber of gram atoms of oxygen in the anhydrous for- mula, and S is defined as S=2 Weight percent oxide i/formula weight oxide 1' per gram atom oxygen. If the analyses are exactly right and the minerals are stvoichiometric, then the sums ought to be 100 percent, except for the unresolvable uncertainties of the oxidation state of iron in the mineral. (The cal- culations assume ferrous oxide.) As the data in table 3 show, many of the sums fail to add up to 100:1 percent. Realistic estimates of analytical uncertainties lead me to include all data having 100: 1.5 percent; for some minerals, a cutoff of :2 percent was accepted. Many of the spots yield- ing poor sums have been reproducibly replicated; wave-length scans of selected spots rule out omis- sion of major elements having sufficient atomic num- bers to be excited in the detectable region. Because all the analyzed spots were chosen to avoid inclu- sions and poor polish and tend to be away from edges and because, wherever possible, only coarse grains were used, the defective sum-s may be real; therefore, they are retained, although they are not at present explained. A few of the analyses tabulated do violate the :2 percent limit. They are retained because the data are in sets showing the compositions of adjacent coexist- ing grains of different minerals. (Such sets are noted in the table.) Thus, because the determinations were made at the same time, the relative proportions of the elements are presumably correctly depicted even though the absolute values may be subject to doubt. I could have swept the problem under the rug by showing the results only as variation diagrams; however, the original data cannot be recovered from variation diagrams, and I felt that presentation of MINERALOGY AND MINERAL ASSEMBLAGES 7 the original data, warts and all, is much to be pre- ferred. Despite the fact that the data were collected on two different micropro-bes (the State University of New York, Stony Brook, probe, A. E. Bence, ana- lyst, and the US. Geological Survey, Reston, probe, N. L. Hickling, J. G. Hammarstrom, and E-an Zen, analysts), the results do not appear to be biased thereby. The analyses identified by six-digit num- bers were made on the Stony Brook probe; the others were made on the Geological Survey’s ARL— SMX 1 probe. In all, about 400 thin sections were examined and about 600 X-ray patterns of rock samples were taken to obtain the data on mineral assemblages. About 30 of the rocks have had one or (more commonly) more minerals analyzed by the microprobe. MUSCOVITE Muscovite is present in almost every rock studied. The only exceptions are a few marble beds of high purity and one or two quartzite beds that contain garnet, magnetite, cummingtonite, and trace amounts of biotite. Certainly every pelitic rock con- tains muscovite as an important mineral; so, unless otherwise stated, it will be regarded as ubiquitous. Texturally, it is most commonly part of the matrix, the plates lying in the plane of the principal foliation of the rock. Intergrowth of muscovite with biotite is not uncommon; intergrowth with chlorite is rarer and is observed mostly in microporphyroblasts in fine-grained, low-grade rocks. In some rocks where the foliation has been crenulated, muscovite lies oriented along both foliations; however, microprobe data do not suggest any significant compositional dif- ferences based on orientation. Microscopically, iron-free biotite (phlogopite), paragonite, and pyro-phyllite could be confused with muscovite. Paragonite and pyrophyllite are readily distinguished by their X-ray diffraction patterns if they are present in more than a small part of the rock. Paragonite has been found in a few samples of low-grade to medium-grade rocks but is never quantitatively important; it is never found in the absence of muscovite. Pyrophyllite has not been seen. Iron-free phlogopite has basal spacings similar to those of muscovite, but the (001)/ (002) intensity ratio is very different, and it can be identified by this means if little muscovite is present. Fortunately, iron—free phlogopite occurs exclusively in metamor- phosed marble; in pelitic rocks, where the biotite is 1Any use of trade names and trademarks in this publication is for de~ scriptive purposes only and does not constitute endorsement by the US. Geological Survey. a normal, iron-bearing, green-brown pleochroic vari- ety, the danger of confusion with muscovite is small. Table 3, section A, gives the microprobe analyses and atomic proportions of 88 muscovite spots from 23 rock samples. These analyses show that most of the muscovite contains minor amounts of iron and magnesium, though one sample (1169—1) contains these components in significant quantities. The anal- yses also show a persistent though small deficit in alkali. Computation of the mineral formulas for mus- covite is made uncertain by this factor, though ad- mixture of phase impurities can probably be ex- cluded. One major problem is the oxidation state of the iron, as it could enter the muscovite structure by Al=Fe3+ substitution, by Fe2+ +Si=2 Al substitu- tion, or by 2 Al=3Fe2+ substitution. Decision on this assignment of an oxidation state depends in turn on how much one is willing to accept the accuracy of the aluminum determinations. Because iron is present in small quantities and aluminum in large quantities in muscovites, a small error in the deter- mination of aluminum could lead to major relative changes in the assignment of iron. Most of the analyses show small deficiency in the alkali content when computed to a basis of 11 oxy- gens (anhydrous formula). Much of this deficiency may be real and is well known to those who study mica (Eugster and others, 1972, p. 161; Guidotti, 1973, and written commun., 1973). The possibility that alkali may have volatilized during the micro- probe analysis was considered, but test runs on the microprobe show that this factor cannot be a major source of the observed deficiency. The data in table 3, section A, show that Within limits of analytical uncertainty the 2:1-layer com- position of muscovite in rocks of different metamor- phic grades and (or) assemblages varies no more than that of muscovite in different spots in the same rock. For example, muscovite in chloritoid- and staurolite-bearing samples does not differ sensibly in aluminum content and in iron-magnesium content from muscovite in hornblende-bearing samples. These conclusions are contrary to those of Guidotti (1973). To a first approximation, therefore, I as- sume that muscovite from the study area is a di- octahedral mica having the ideal composition (K, Na)Al,Si3010(OH)2, and this ideal composition is used in calculations in a later section of this report. The muscovite analyses plotted in figure 2 indicate that most of the muscovites are low in phengite. The one major exception to this generalization is the muscovite from sample 1169—1, which is a graywacke in the Everett Formation on the low-grade western 8 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. ‘ Ferriphengite 20 40 Phengite 20 Ferrimuscovite \4/IL 60 V _ , / o 40 50 60 Muscovute 70 80 Alvi _> FIGURE 2.——A1‘”—Al”—(Femm+Mg) plot of muscovite analyzed by use of the microprobe, in atomic proportions. Each num- ber denotes how many samples plot at the particular point. The end members muscovite, ferrimuscovite, phengite, and ferripheng'ite defined by Ernst (1963) are also plotted on the main diagram. The small triangles represent muscovite in sample 1169—1. side of the area (table 2 and fig. 2 (triangles)). I Three wet-chemical analyses of muscovite sepa- F‘our analyses (table 3, section A) of the muscovite l rates are given in table 4. Samples 355—1 and 356—1 revealed a uniformly high phengitic content, as de— are coarse-grained staurolite-garnet schists; min- fined by Ernst (1963). era] separation was a straightforward procedure, Because musc‘ovite is nearly ubiquitous in pelitic ! and the chemical analyses show the muscovites to rocks, it has been used widely as a projection point I be nearly identical and to conform to an ideal for- for depicting the mineral assemblages (Thompson, mula, with substitution in the alkali position. Alkali 1957). If, however, the muscovite can be far from is weakly deficient and is comparable with alkali its ideal composition, use of this phase for projec- 1 contents indicated by most of the microprobe data. tion must be evaluated in each study. I Sample 140—2 is a fine-grained chloritoid-chlorite MINERALOGY AND MINERAL ASSEMBLAGES 9 phyllite, and separation of the minerals was diffi- cult; some minor mixture of quartz was possible, though mixture of chlorite was considered unlikely on the basis of a visual examination of the separate and its X-ray pattern. The analysis indicates that sample 140—2 is about 10 percent deficient in alkali and has some excess silica. If we assume an im- probably high upper limit of 5 percent by weight of quartz impurity, the atomic silicon content would be 3.0, but the alkali content would be increased only to 0.93. Thus, the alkali deficiency may be real. I had hoped that a comparison of the wet—chemical analyses, which distinguished ferric and ferrous iron, could provide the basis for the evaluation of total iron in the microprobe analyses. All three wet analyses of muscovite show the ferrous iron/ magnesium atomic ratio to be on the order of unity and the sum of the ferrous iron and magnesium. cations to be a few hundredths of an atom per 11 oxygen; these micas are thus very weakly phengitic. Lumping of ferric and ferrous iron would change this ratio, of course, but not so much as to make the results incompatible with the range of ratios shown by the microprobe data. In the absence of criteria based on consideration of atomic site occupancy, such as exist for the amphibole minerals (Papike and others, 1974), the problem of partition of oxi- dation states of iron in the microprobe analyses of muscovite must remain unresolved. PARAGON ITE Paragonite is found in some aluminum-rich pelitic rocks but not in large enough quantities that it can be physically separated from the other minerals for detailed study or can even be studied by the micro- probe. The presence of this mineral is generally inferred on the basis of one or more weak X-ray diffraction peaks of the (001) set superimposed on the much stronger basal reflections of muscovite. Thus, the presence of paragonite was easy to detect, but its composition, including possible intergrowth with the phengitic sodium-rich mica (Laduron and Martin, 1969) , could not be determined. The mineral assemblages including paragonite are given in table 1. The rocks run the gamut of the entire metamorphic range in the area, and the assemblages are entirely consistent with available published data (Zen and Albee, 1964; Eugster and others, 1972). The presence of paragonite indicates that the muscovite is saturated with the sodium component and that the assemblage lies on a section of the multicomponent solvus between the muscovitic and paragonitic end members. The basal spacing and chemical composition of muscovite can be de— termined with greater certainty than those of pa- ragonite, which is less abundant; knowledge of the muscovite composition allows estimation of the met- amorphic temperatures on the basis of the solvus calculated by Eugster and others (1972). BIOTITE Biotite is a very common mineral in the pelitic assemblages above the biotite zone; by definition it is not found below that zone. Biotite typically forms stout books lying either within or across the con- spicuous surfaces of ‘penetrative cleavage. Inclusions of zircon are common and show pleochroic halos. Dusty inclusions are also found, frequently preserv- ing earlier foliations. Biotite is commonly inter- grown with muscovite; microprobe data on inter— grown pairs are noted in table 3. Biotite inter- growth with chlorite is also common, though in some rocks the chlorite probably resulted from alter- ation of biotite. The two minerals are probably equilibrium phases where they are in fresh and sharp intergrown contact along the basal cleavage or where they are present as discrete unaltered crystals in the rock. With rare exceptions, biotite in the pelitic rocks is pleochroic from brown to light olive brown; only rarely is it pleochroic in shades of green. In lime- silicate rocks, the biotite is commonly a phlogopite showing faint light tan to colorless pleochroism. The 60 microprobe analyses (table 3, section B) show a remarkable uniformity in the general com- position of the biotite. The values of Aliv per 11 oxy- gen (anhydrous formula) are within the range of 1.17—1.45, and the mean value is 1.33: 0.05 (stand- ard deviation). The AlVi value is slightly more variable, the mean and standard deviation being 0.40:0.09 (range, 0.18—0.59). The aluminum con- tent exceeds that for the formula suggested by J. B. Thompson, Jr. (oral commun., 1955), which is K (FengyMn) 2.67AIVi0.3asi2.67Aliv1.33010 (OH) 2 = 1/3 (K3(Fe,Mg,Mn)sAlvi(S‘i8Al4iV)030(OH)6). The ma- jor deviation is an excess of about 0.1 in Alvi per 12 oxygen and is almost precisely compensated for by a ubiquitous deficiency of potassium and sodium; the mean value of the sum of these two elements in the same analyses amounts to 0.88:0.05. How- ever, individual analyses do not reveal this nice correlation of excess Alvi and deficient alkali; only the statistical mean values do, and the correlation is a weak one. In addition, one biotite separate (356-—1) was I also analyzed by the wet-chemical method (see table 10 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. 4); this same biotite was used for dating by the K-Ar procedure (Zen and Hartshorn, 1966). Nine microprobe analyses were made on spots of biotite crystals immediately adjacent to or in parallel growth with muscovite. The average Al”, A1“, and alkali contents of these biotites are, re- spectively, 1.35:0.03, 0.45:0.05, and 0.87:0.06 and are identical with those of the bulk rock. Another set of 11 analyses is for biotite in im- mediate contact with chloritoid. For these, the re- sults are AliV=1.35-l_-0.02, Al“=0.40:0.03, and alkali= 0.87:0.04. These results are not distinguish- able from the average or from the results of analy- ses of biotite in immediate contact with muscovite. A set of 21 analyses is of biotite in assemblages that include staurolite, and the results are A11V= 1.34:0.04, Al“=0.40:0.07, and alkali=0.88:0.07. These are not distinguishable from the mean. Val- ues of the last set are in full accord with those for the few individual analyses of biotite in the imme- diate vicinity of staurolite crystals. In contrast, five analyses of biotite in hornblende- bearing assemblages give the values Al”=1.29i 0.03, AlVi=0.33iO.07, and alkali= 0.87:0.05. Seven 2 analyses of biotite in rocks including an amphibole 2If six other analyses of biotite associated with amphibole that were rejected for other reasons were included, the results would be unaffected. flto A .50 as a phase (samples 102—1, 289—2, and 487—2—4) give the values of AliV=1.26:0.05, Alvi=0.33:0.06, and alkali=0.88:0.05. These values suggest that these biotites have a lower aluminum content, espe- cially octahedral aluminum content, than those in the above-mentioned three groups. There is no cor- responding effect on the alkali content within the limits of uncertainty. The biotite analyses are plotted on an AFM dia- gram (fig. 3), where major deviations from the tight clustering result from two amphibole-bearing rocks. CHLORITE Chlorite is a common mineral in the pelitic rocks and coexists with every other mineral of the area. It is part of the groundmass of the rock, develop- ing parallel to the foliation or foliations. It is also present as larger plates, especially in sandwiches having ilmenite centers (fig. 4) ; generally, the plates are oriented at a sharp angle to the conspicu- ous foliation plane. Chlorite interleaved with biotite and muscovite may be an alteration product (especially of biotite) , but its origin is difficult to determine. If the inter- leaving is poorly defined, if chlorite is spatially al- ways associated with biotite, and, especially, if the to AN .50 ++22 +43++ ++22+ + 2+ ++2+ + +4 + oo++ ++- ++ .+ + ... + + . C C v _ y M F .10 .20 .30 .40 .50 .60 .70 .80 .90 FIGURE 3.—At0-mic A(Al-alkali.s)FM compositions for biotite samples analyzed by use of the microprobe. Scales are shown in decimal fractions rather than in percent. The samples from assemblages carrying an amphibole are separately identified by dots. A cross (+) represents a single analysis of amphibole-free rock. A number instead of a symbol indicates how many analyses (all such analyses are of samples from amphibole-free rocks) plot at that point. The “ideal” biotite of formula H2K(Fe,Mg)gAlSi3012 plots along the base line. The biotite composition suggested by J. B. Thompson, Jr., lies along the line A:0.2. All the analyzed biotites from amphibole-free rocks plot above this line. MINERALOGY AND MINERAL ASSEMBLAGES 11 FIGURE 4.—Photomicrograph of sample 376—1 (cross-pOIarlzed light), showing ilm‘enite (Ilm) sandwiched between chlorite (Ch) plates. Also shown is a bent (though not broken) ilmenite lath and stresseinducedfl) twinning in a zoned porphyroblast 0f plagioclase (Pg). chlorite shows radiation halos around inclusions of zircon, which are common in biotite, then the chlo- rite is considered to be an alteration product. Such chlorite commonly is also sufficiently inhomogeneous even Within a single platelet to show distinct patchi- ness in color in plane light and in the anomalous color between crossed nicols. Oxidized chlorite is found in a few rocks but is generally in such small amounts that its presence can be established with certainty only by micro- probe analyses. However, such chlorite tends to be strongly pleochroic in murky yellow and brown and lacks the luster or the “birds-eye maple” tex- ture of biotite under the microscope. Resemblance of oxidized chlorite to stilpnomelane is strong, and distinction is not easy. Chatterjee (1966) has dis- cussed the common occurrence of oxidized chlorite in many pelitic rocks. Table 3, section 0, lists the results of 51 micro- probe analyses of chlorite. All samples show an aluminum content in excess of that given by the ideal Pauling (1930) formula. (For reference, the Fe,Mg/A1 ratio for an ideal Pauling chlorite is 2.5; for a chlorite whose ratio is equal to that of alman- dine, the value is 1.5.) Two chlorite samples were separated for wet- chemical analysis; the results are given in table 4. Sample 355—1 was a clean separate that could have been contaminated only by traces of muscovite, and the small amount of alkali was so calculated. Sample 140—2 was very fine grained, and contamination was more likely. The phosphorus is probably from apatite, and the alkali from potassium-sodium mus- covite (the ratio of these two elements in the analy— sis of chlorite from sample 355-1 very nearly cor— responds to that in the analysis of muscovite from this rock; see table 4). After subtracting the im- purities, one could calculate the chlorite formula in several ways, especially by considering the iron to be in different oxidation states. However, the differences among these calculated chlorite struc- tural formulas are so slight as to suggest that con- tamination or the assumption regarding the oxida- tion of iron have little effect on the main features of the formula. All these chlorite formulas have distinctly high aluminum content in both the four— and six-fold positions, as do formulas for chlorites in other pelitic rocks (Zen, 1960). CHLORITOID Chloritoid is present in pelitic rocks showing a wide range of metamorphic grades. It is in both the Walloomsac and the Everett Formations. In the lower grade rocks, its presence is indicated by tiny dark-green lustrous specks (Zen, 1960). In the higher grade and generally coarser rocks, it forms dark greenish-black crystals superficially resem- bling biotite but having a duller luster and more brittleness. It could be confused with platy ilmenite; a magnet helps to distinguish them. In thin section, chloritoid is found typically in lath-shaped crystals, commonly showing simple to polysynthetic twinning; the composition plane is parallel to the long direction (in the (001) plane), and extinction positions are at most only slightly different from the trace of the composition plane. The crystals most commonly have straight, regular sides parallel to (001), but the terminations are commonly ragged. In some crystals, inclusions of tiny grains, especially quartz, give the mineral a worm-eaten appearance, though crystal outline can almost always be recognized. The pleochroism in chloritoid is from bluish-green to straw yellow, though some crystals have little or no pleochroism. The different pleochroic colors are not reflected in the major chemical composition determined on the basis of the microprobe data. Microprobe data on 59 chloritoid analyses are given in table 3, section D. Despite the fact that the mineral is found in rocks of different metamorphic grades, the composition is remarkably uniform. Chloritoid has one atom of silicon and two atoms 12 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS, CONN., N.Y. of aluminum per six oxygens (anhydrous formula). There is no need to assume or evidence for ferric iron substitution for aluminum. Four analyses from a single rock from below the garnet zone (sample 140—2) gave Fe/(Fe+Mn+Mg) =0.86:0.01. Six- teen analyses of chloritoid from samples 338—1, 466—1, 506—1, and 509—1, from the garnet zone and the staurolite-garnet transition zone gave the same ratio as 0.85:0.01. Ten analyses from the staurolite zone but in rocks without that mineral gave the ratio as 0.86:0.01. Thirty-five analyses of chlo- ritoid in the matrix and coexisting with staurolite gave this ratio as 0.85:0.02. Two analyses of a single sample from the staurolite zone in which the chloritoid is found only as inclusions in the centers of garnet gave this ratio as 0.83:0.01. For the present, we must assume the chloritoid to be constant in composition regardless of grade and mineral assemblage; the slightly lower Fe/(Fe+ Mn+Mg) ratio of the included chloritoid does not seem significant, because in several other samples, chloritoid within and outside garnet gave identical chemical analyses. Similarly, an effort was made to see if chloritoid inside garnet has higher manganese content, as cores of garnet commonly are manganese rich. The data of table 3, section D, do not show such a trend. The chloritoid of sample 140—2 has a higher manga- nese content than do those of others (about 0.04 atoms of manganese per six oxygen, anhydrous for- mula) ; this could well be because the rock is below the garnet zone. In rocks containing garnet, the chloritoid is depleted of manganese, and this low manganese content is maintained in rocks of higher grades. STAUROLITE Staurolite is a common mineral in the higher grade metamorphic pelitic rocks. The assemblages that include this mineral are given in table 1. Fifty- eight electron—microprobe analyses of staurolite from 13 different rocks are given in table 3, section E. In addition, two separates of staurolite from two different assemblages and different stratigraphic units (355—1 from the Everett Formation and 356—1 from the Walloomsac Formation, both from near Lions Head) were analyzed by the conventional wet- chemical methods; the results are given in table 4. Interpretation of the microprobe analyses is un- certain for several well-known reasons. The oxida- tion state of iron is uncertain. Because iron is such a major component, even a small error in analysis could lead to significant differences in site assign- ment. The amount of structural H20 in staurolite has been debated since the reexamination of the staurolite structure by Naray-Szabé and Sasvari (1958). These workers suggested an ideal end-mem- ber formula of HFe2A198i4024. However, Richardson (1968) synthesized staurolite “on composition” with an initial mix whose anhydrous formula corresponds to FezAIQSimsozg; he thus advocated a formula con- taining 2 hydrogen per 24 oxygen. Griffen and Ribbe (1973) supported this formula, largely on the basis of three direct determinations of densities of analyzed material and of an assumption that no ferric iron was present. Fed’kin (1975) recently reviewed the problem of staurolite composition. Of the two wet—chemical analyses, the one of specimen 355—1 was made with special care. The result indicates minor substitution of ferric iron for aluminum (less than 0.3 out of 9 aluminum sites, and 17 atomic percent of total iron as ferric iron). The amount of H20, which was determined by a microcombustion train on 200 mg of sample using V205 flux, was corrected for H20— (110° C), and the resulting H20+ gave 1.5 hydrogen per 24 oxy- gen, exactly halfway between the two formulas. Sample 355—1 was also studied on the automated microprobe; the standard was repeatedly redeter- mined during the analyses. The results, given in tables 3, section E, and 5, show excellent agreement with the wet-chemical results. Addition of one mole of H20 per 23 oxygens (reckoned in the anhydrous formula) to the anhydrous sum would have added 2.23 weight percent H20 and brought the total of the probe result to 101.0 percent (without correct- ing for ferric iron, which would have added to the sum). Addition of 0.5 mole H20 per 23.5 oxygens to the anhydrous sum would have brought the sum to 99.9 percent. Either sum is acceptable; thus the results do not help to resolve the problem, though the less hydrous formula seems more appealing. ,An- other discriminant, though in practice a poor one, is to calculate the number of cations per 23.5 oxy- gen. The Naray—Szabé and Sasvari formula predicts 15 cations (exclusive of hydrogen), but the Richard- son formula predicts 15.07. The 58 probe analyses gave a mean of 14.96:0.06, and though far from definite, the result does seem to support the idea of a less hydrous formula. Without further informa- tion and with the indication from Mossbauer spec- tra (Dowty, 1972, fig. 1) that a slight amount of ferric iron may be in the sample, I accept for the subsequent calculations the less hydrous formula of staurolite proposed by Naray-Szabo and Sasvari (1958). MINERALOGY AND MINERAL ASSEMBLAGES 13 Dowty (1972) also found that the staurolite of sample 355-1 shows no evidence that ferrous iron occupies the six-fold position. Hollister (1970) studied an aliquot of the same sample for the pos- sibility of sectorial zoning and concluded that tita- nium is preferentially sited on the (010) growth face relative to the (100) face by a factor of 1.56 but that no evidence exists for zoning among alu- minum and silicon. Holliste-r (1970) also studied the staurolite from sample 356—1, which showed sectorial zoning in titanium by a factor of 1.19 in the same zone and no zoning in aluminum or silicon. Albee (1972) discussed the problem of correlat- ing wet-chemical analyses of staurolite with micro- probe data. In addition to the problems outlined above, staurolite is a particularly difficult mineral to analyze by the conventional methods because of its refractory nature; moreover, tiny inclusions are common in staurolite, and these may be very difi‘i- cult to eliminate. The data of table 5, on the other hand, seem to indicate that with sufficient care an acceptable agreement between these two methods of anaylses is attainable. Despite Hollister’s (1970) data suggesting minor sectorial zoning, staurolite is generally not visibly zoned. The microprobe data also indicate that stau- rolite is considerably more homogeneous, both be- tween grains and within grains, than are garnet or hornblende. staurolite is, however, characteristi- cally rich in mineral inclusions as well as trains of dusty particles; these may define a foliation trend that may be at angles to the foliation trend in the groundmass and that I interpret as rotation after growth. Some staurolite crystals have rims rela- tively free of inclusions, yet others show excessive incorporation of the groundmass, mainly quartz, in quartz-rich and mica-poor rocks, so the grain boundaries are poorly defined. In garnet-staurolite assemblages, the garnet com- monly contains chloritoid and ilmenite as inclusions. Interestingly, staurolite never has such inclusions even though almost certainly a paragenetic rela- tionship exists between staurolite and chloritoid. The common presence of inclusions in garnet is correlated with its persistent compositional zoning and indicates lack of recrystallization; staurolite, despite its refractory nature, may recrystallize more readily. The wet-chemical analyses of both stauro- lites show significant zinc, a fact confirmed by the microprobe data on the same samples and by deter- minations of zinc in staurolite from other localities (Hollister, 1970). GARNET Garnet appears in pelitic rocks throughout the eastern and southern parts of the area, and its first detection marks the lower limit of the mapped garnet zone. It persists as part of the garnet-stauro- lite assemblage in the highest grade rocks in the area and as garnet-sillimanite rocks in the Canaan Mountain Schist of Agar (1932, 1933) in the area immediately southeast of the study area. In mica-rich rocks, garnet is euhedral, although commonly the crystals are full of inclusions consist- ing of quartz, ilmenite, chloritoid, some rare mica flakes, and trains of dust. Plagioclase inclusions have not been positively identified. Helicitic texture has been observed, though not commonly; nothing resembling the elegant “rolled garnets” described from eastern Vermont (Rosenfeld, 1970) is found in the study area. Some euhedral garnets are broken, a fact that can be established by observation of trun- cated crystal outlines or of truncation of zoning or by distinctly nonsymmetrical composition profiles determined by microprobe analyses across the crys- tals. Zoning in garnet is best established by micro- probe data (fig. 5). Zones are accented by concen- tric banding of color and by the zonal arrangement of inclusions. The rim is commonly nearly inclusion free. Zoning or lack thereof is by and large char- acteristic of a given rock and thus appears to record accurately the history of crystal growth. In quartzite or very quartz-rich pelitic rocks, garnet does not tend to become euhedral and sub- equant as it does in a more pelitic matrix. Rather, it commonly forms anhedral, anasto-mosing crystals in the interstices of quartz grains, thoroughly “wet- ting” the interface. Garnet in quartzite that has a distinct planar fabric may form long stringers. These stringers are not zoned; for instance, micro- probe analyses indicate that specimen 161—1 (table 3, section F) is homogeneous. The present shape of the crystals is not the result of simple deforma- tion, because these stringers show no evidence of crushing or even of fracturing; if crushing was the initial cause of the shape, then the crystals must have been thoroughly recrystallized subsequently. Table 3, section F, gives the microprobe analyses of 90 garnet spots in different mineral assemblages. Table 4 gives the wet-chemical analyses of two gar— nets from two rocks 1 km apart having different mineral assemblages: Sample 355—1 from the Ever- ett Formation is a garnet-staurolite—chlorite-plagio- clase-muscovite-ilmenite-quartz rock; 356—1, from 14 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. ,MnO (0), M90 (o), CaO (A) WEIGHT PERCENT Fe0(tota|) I?) WEIGHT PERCENT FeO(tota|) (x) A 4° xl I I I | I laT I I I I I I I I I I I I I I I X X X 38— x xx x x xxx_ X X X X XX 36— _ X xx Xx X Xxx X xx)< 34— " X X ~ X X xxxx Xx x 32— _ e e 6— a o 0000 0000 000000 4— A A468: AA AAA AAA AAZE __ A AA A O A AAA A 8éAAA AA A O A 24 3 0, 0 ’3’. ch O .03. . . . 0;. A O o O . °.°o 00......0 o 0000'. . 0 000 o 00 0 I I I I I I I I L I I I I I I I I, I I I I T I 0 2 4 6 8 1012 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 DISTANCE, IN 100—,um INCREMENTS B 43 I I I I I I I I I I I I I I I I I I I ‘4 42— xx x xxxx —13 X X 41— x x ." X " ~12 x x 1? x X x x V 40— X x —11% x x x o U 39— x O —10.; " x O 0 L5 38— x ‘9 O U) o E 37— 0 0 —8 s E 36— —‘7 o E 35— x —6 '2 34— O —5 8 E 33— #4 n. O .— A I 32— 0 2AA AA: 43 g 00 A x 0 31— 00 o —2 E 0 0A 0 x o _ 8.. . 0 g _ 3° ' °o§8:““°azz;'- .x ;;°:§ 288229" 29 AIM I" ‘I I I I ‘I‘ T I”? I I ‘I‘ I ‘I ‘IA‘I‘AM o 0 2 4 6 8 1O 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 DISTANCE, IN 25 _,u.m INCREMENTS FIGURE 5.—Compositi0n profiles determined by microprobe analysis across garnet crystals in (A) sample 356-1, in 100mm increments, and (B) sample 487—2—4, in 25mm increments. MINERALOGY AND MINERAL ASSEMBLAGES 15 the Walloomsac Formation, has the same assem- blage plus biotite and epidote but has only trace chlorite. These analyses are necessarily on bulk separates and thus are “averages” of the zoned garnet. The garnets are all almandine rich. They show variable amounts of manganese, which is enriched in the cores of garnet but decreases progressively outward, in accord with well-established patterns (fig. 5). Of considerable interest is the fact that calcium as the grossularitic component is found in small but significant quantities in all the garnets probed. Calcium zoning is not as marked as manga- nese zoning. The calcium content of garnet seems to increase either toward or away from the rim in different crystals, but some calcium is generally present in the rim (Guidotti, 1970). The calcium contents of garnet from hornblende-bearing rocks are notably high; calcium may occupy as many as one-fourth of the total divalent cation positions. The ubiquity and sensitivity of calcium in garnet in pelitic rocks may have more petrologic signifi- cance than has generally been thought, and this topic is considered in other sections. Several rocks contain garnets that show unex- pectedly high contents of ZnO in a few point analy- ses. The location of zinc in lower grade rocks is unclear. Albee (1972) suggested that zinc may be concentrated in muscovite. My analytical data do not show much zinc in muscovite; a few biotite analyses do show ZnO. The analytical data (table 3, sections K and L) indicate that zinc is not prefer- entially concentrated in lower grade rocks in mag- netite or ilmenite. The concentration of zinc in gar- net is intriguing and needs more comprehensive study and corroboration. KYANITE Agar (1932) reported kyanite from the “Salis- bury Schist” in the present study area but gave no details. Despite careful search, I have found kyanite in only one suite of samples, which is from a,hillock on the west bank of the Housatonic River at the southeast edge of the study area and thus is from the area of highest metamorphic grade. The mineral assemblage is kyanite-staurolite-garnet-biotite—mus- covite-plagioclase-quartz-tourmaline-ilmenite. Four different samples from this hillock were examined; two (including sample 655—1—1) contain kyanite, and only one sample, which is free of kyanite, con- tains chlorite, obviously of secondary origin. Chlo- rite and kyanite do not seem to form a compatible pair in the presence of staurolite and biotite here. Microprobe analyses of the kyanite are given in table 3, section G. AMPHIBOLES Three kinds of amphiboles have been found in the area: tremolite-actinolite, cummingtonite, and high-aluminum hornblende. TREMOLITE Tremolite is found in metamorphosed impure dolostones of the Stockbridge Formation, especially in unit a3 (Zen and Hartshorn, 1966), where it often grew in clusters around quartz nodules (which could be metamorphosed chert). Table 4 gives a complete analysis of a colorless tremolite from this unit. The analysis indicates that the tremolite has a Fe/(Fe+Mg) ratio less than 0.007. Tremolite commonly forms laths and clusters of laths ranging from a few millimeters to several decimeters in length. Mineral assemblages that in- clude tremolite are given in table 1 (samples 598-2, 659—1, 677—1, 1054—1). CUMMINGTONITE Cummingtonite has been found in three outcrops of quartzose beds in the Everett Formation (tables 1 and 2, samples 170—1, 170—2, 487—2). The mineral is present as radiating sheaves of long laths a frac- tion of a millimeter across (fig. 6). As seen under the microscope, these laths are colorless; individual laths are as much as 0.1><0.5 mm. Microprobe data on two crystals from sample 487—2—4 are given in table 3, section H. These data may be compared with data on cummingtonite by Stout (1972). The analyses show a near absence of calcium, sodium, and aluminum and very low fluorine (about 1 per- cent of the OH sites). The Fe/(Fe+Mg) ratio is about 5/7, so these crystals may be properly called grunerite. The assemblages associated with the cummingtonite include an almandine—rich garnet, biotite, magnetite, quartz, and traces of muscovite and chlorite. HORNBLENDE Hornblende is known from three localities in the study area: two in the Walloomsac Formation (samples 102—1 and 289—2) and one in the Everett Formation (sample 161—1). All three hornblende samples are highly aluminous (A1"i occupancy about 1.5/23 oxygen, AliV=2). The crystal chemistry, prob- able oxidation ratio of iron, and mineral assem- blages of samples 102—1 and 289—2 are fully dis- cussed elsewhere (Doolan and others, 1978). 3 Mapped at least in part as Unit B by Burger, 1975. 16 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. FIGURE 6.—Photomicrographs of cummingtonite (Cum) as- semblages for samples (A) 170-1 (plane-polarized light) and (B) 487—2 (plane-polarized light). Other minerals present are chlorite (Ch), biotite (Bt), and garnet (Ga). Doolan and others (1978) also presented the optical data on one hornblende (sample 102—1). Both hornblendes from samples 289—2 and 102—1 are pleochroic from "grass green to greenish yellow. Hornblende from sample 161—1 is pleochroic from blue green to grass green, has the highest Fe/ (Fe+ Mg) ratio, and has the lowest calcium content (though sodium contents in the M(4) sites of all three samples are about equal). Sample 161—1 is of the lowest metamorphic grade (just above the gar- net-zone marker), 102—1 is just within the stauro- lite zone, and 289—2 is well within the staurolite zone. On the scale of microprobe analysis, the horn- blende crystals are not uniform in composition, either between grains or even within an optically homogeneous single grain (Doolan and others, 1978; see also table 3, section H, and fig. 7). The within- grain variations are smaller than the between-grain variations for the same sample (on the scale of a single thin section used for microprobe analysis). The variations cannot be related to zoning, to prox- imity to different phases, or to the stratigraphic unit. Perhaps the inhomogeneities can be eventually attributed to growth factors analogous to the sec- torial zoning in andalusite and staurolite described by Hollister (1970); they could be a good fossil record of the scale of chemical equilibrium during metamorphism. EPIDOTE Epidote is a subsidiary mineral in some mineral assemblages, primarily those in the lower grade rocks (table 1). In lower grade rocks, the relatively high refractive indices of epidote make it conspicu- ous. Epidote in these rocks is colorless to faintly yel- low and is pleochroic in this range; it commonly is present as apparently subrounded clusters, a milli- meter or so across, that are aggregates of finer sub- hedral crystals. In higher grade rocks (garnet zone on up), epidote is present as small, discrete, sub- hedral to euhedral crystals. The textural relations of epidote in lowest grade rocks show the mineral aggregate to crosscut rock foliation; thus, the mineral is nondetrital. The rea- son for the rounded aggregate shape is unclear; it may be pseudomorphous after some preexisting min— eral or rock fragment, possibly limy microno-dules. The highest grade rocks that contain epidote are samples 356—1 and 590—1, both schists of the Wal- loomsac Formation found well into the staurolite zone (table 2 and fig. 1) . The mineral assemblages of both include staurolite-garnet-chlorite-epidote—bio- tite—muscovite—plagioclase-quartz (table 1). These are the only instances of coexisting staurolite and epidote found in the area (fig. 8). The photomicro- graph (fig. 8A) shows these euhedral minerals to be in mutual contact; presumably they are equilibrium phases. The microprobe analyses of epidote are given in table 3, section I . These analyzed epidotes all contain appreciable iron, which on the basis of stoichiometry should be computed as ferric iron substituting for WEIGHT PERCENT SiOz I.) WEIGHT PERCENT MgO (A), CaO (V), N32O (D) FIGURE 7.—Composition profiles determined by micro- probe analysis across a hornblende crystal in sample MINERALOGY AND MINERAL ASSEMBLAGES 42 | . o '0 o _ 40 '— . .. o o 0 o 00 . .0 . I 0 o oo' 0 o. . o 38 — _ a o .. 36 — —‘ o 34 — . _ 32 14 25 O 12 — v v V v v — 24 v VvVv v V v vvv VVVvv W V V v V v v V v Vv 10 — — 22 o 0 V o 0 00 0° 0 o 0 V o 0 0° ° °° ° ° 20 8 _Y"xxxxxxo° 0° 0xo 0 XA X x x 39x 66 Oé x x ox x AAA :8 x A x A 6 — x A — 18 A AA A AAfi AA)( A A A A AAA A A x X AAA A A A x X A x 4 — xx x _16 2 _ ~ 14 E El D DUI] a DD EH: DD ,3 5cm DUDE! DEED a CI Dug D DUDE} D 0 I 1 I 0 10 20 30 40 DISTANCE, IN 5—me INCREMENTS 102—1, in 5-,um increments. WEIGHT PERCENT A|203 (x), FeO(tota|) (0) 17 FIGURE 8.—Photomicrographs of coexisting staurolite (St) and epidote (Ep) in (A) sample 356—1 (plane-polarized light) and (B) sample 590—1 (plane-polarized light). Other minerals present are garnet (Ga), biotite (Bt), and mus- covite (Mu). aluminum (pistacite end member). The extent of this substitution is very nearly constant at about 30 percent of the aluminum positions. PLAGIOCLASE Plagioclase is nearly ubiquitous in the pelitic rocks and is the only feldspar found in most. Potassic feldspar is found only in a few granulites and car- bonate rocks, chiefly in the basal part of the Dalton Formation (Precambrian? and Cambrian) (tables 18 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. 1 and 2). The plagioclase is present in a variety of habits: 1. Plagioclase forms microporphyroblasts as much a 1 mm across. Such feldspar is readily recog- nized in hand specimen as White spots scat- tered throughout the rock. Because the grains all have their long dimensions alined parallel to the rock cleavage surfaces, the particles on these surfaces commonly appear as if smeared out. In thin section, these crystals generally show rude crystal outlines. These crystals are especially well developed in relatively low grade pelitic rocks below the garnet zone, Where the other phases are in fine grains but the feldspar is relatively large. These plagio- clase crystals are commonly zoned, the center having less calcium than the outer zone. Many porphyroblasts in samples from the part of the study area near the “retrograde” isograd mapped on figure 1, however, show an outer- most calcium-free zone in fairly sharp contact With the calcium-rich zone (fig. 9). In some porphyroblasts, the most calcic and spatially intermediate zones have altered to a mixture of white mica and a very fine grained phase hav- ing a high refractive index; presumably, the phase is zoisite or epidote. Figure 10 gives several microprobe profiles of such triply zoned crystals, as well as a profile of a plagioclase from a staurolite schist for comparison. Be- cause the electron beam used in the traverses across crystals has an effective diameter on the order of 10 ,um, actual change of sodium content is probably sharper than shown. All the plagioclases contain negligible potassium. 2. In higher grade pelitic rocks, plagioclase is pres- ent commonly as xenomorphic crystals. These are either oval crystals in finer ground mass (in which case they show up megascopically as white spots) or are part of interlocking sys- tems of crystals in the rock. Such crystals are readily identified in thin section by their cloudi- ness compared with quartz. Zoning is found but does not show reversal toward albitic rim as in category 1, and the zones are not sepa- rated by sharp boundaries. Many rocks show- ing crenulated foliation defined by mica aline- ment contain plagioclase that has grown into tabular crystals that fill the space defined by the warped folia (fig. 11A). Other crystals in- clude dust trains defining an earlier foliation surface as relict but show no conformity of FIGURE 9.—Photomicrographs of triply zoned plagioclase from (A) [sample 3—3 (cross-polarized light) and (B) sam- ple 360—1 (cross-polarized light), showing the calcium-free rim in sharp contact with a calcium-rich intermediate zone, now partly altered to sausserite, and a less calcium-rich core. Compare with the profiles shown on figure 10. the crystal morphology to the folia configura- tion (fig. 113). 3. Plagioclase forms small crystals that are difficult to recognize microscopically and that can be identified mainly by the X-ray diffraction pat- tern a-nd by microprobe analysis. The most calcium-rich plagioclase found in the area forms small crystals, as in sample 289—2. The atomic Ca/ (Ca+ Na) ratio exceeds 0.9 (table 3, Section J). MINERALOGY AND MINERAL ASSEMBLAGES 19 B I I A “I— . .0 _ __ O" o _ I I I V _ 10 _ 9 . — O O O o . ' o o o o’ o 9 _ _. . o _ 8 _ P . 000 . w _ '"o. ..o I; o o o . .0 3, 8 _ _ <0 — O — N z o o '. Z . o o .4 7 _ oo o . ‘ _ i 7 _ ‘ _ I— . O z " . ‘° 0 LL! 0 o o O o _ _ ,_ 6 — — E E I<—-I— Edge of crystal—v—>- ,_ e p ‘_ g 5 — ° — I <—I————————— Edge of crysta|———~—> Lu (9 a. E 2 — — 4 — — 3 'i ._ _-2o 9 — — 40 _ _ ' Lu 3 _ _ ' c E '_ \++ _ 30 .c 3-; ++‘++ ++++ + < +++++++‘ < j; 1 — ' ‘ — _ I— 2 — + x — I2 0 ‘ , o“ — 10 E — +. — 20 Lu v} Q . Q — + — D: 1 — + ’4‘ ~ 0: ++ +~ ’+ ++ + Lu — ~+ — 10 E + ++++++ + + + , n. + .+ - + +++ + ~+ ++ + \ 0 7 I l . I I d: 0 0 +++ I I ‘ +1 0 100 200 300 '400 o 50 100 150 “DISTANCE, IN urn DISTANCE, IN um C 4 I I I I I I I _— —— 40 b. 3 — I. .0 — 8 .‘o . G“ o .0 l— __C Q. C. .“O ..L_ 30 Z . o . o .0. C (”5' o q. o < E 2 _ ° . . as: '- oo _ '— o 0 Z ‘L _ °. . a ..' "'0 — 20 8 13—: _o 000 “be..- _ D: o O . O O E m 3 1— — a , — 10 O O o I I I I I I I I I . I I 0 200 400 600 800 1000' .1200 DISTANCE, IN um FIGURE 10.—Composition profiles determined by microprobe analysis across zoned plagioclase of (A) sample 3—3, (B) sample 360—1, and (C) sample 14—1, which does not show reversal and is from a staurolite-bearing rock. An, anorthdte component. Special treatment of the analytical data on plagio- | reasonableness of analytical data can be applied with clase is possible because the site occupancy of vari- ous atoms in feldspar is well understood and tests of I some confidence. In order to apply these tests and I avoid excessive round-off errors, the numbers in. table METAMORPHIC MINERAL ASSEMBLAGES, FIGURE 11.——Photomicrographs showing (A) extreme habit adaptation of metamorphic plagioclase to the foliation sur- face of the rock, sample 423—1 (cross-polarized light); (B) late plagioclase including folded foliation defined by dust trains; the sigmoidal fold is in a single crystal, sam- ple 340—1 (plane-polarized light). 3, section J, are carried to more places than are those in other parts of table 3 and than are warranted by the original data. The results of three tests are tabulated in table 3, section J: the number of tetrahedrally coordinated atoms per eight oxygens (Si, Ti, Al) is designated Z; the number of atoms occupying interstices of the ‘ framework (Ca, Na, K, Ba, Mg, Mn, Fe) is desig- nated X; and quantity Y is the difference between the sum of the numbers of each type of X atom mul- TACONIC ALLOCHTHON, MASS., CONN., N.Y. tiplied by its cationic charge and the number of alu- minum atoms. A fourth test, that the sums of cat- ionic charges should equal 16, is really a test of arithmetic accuracy and is strictly obeyed by all the listed data. The role of iron is a problem, as ferric iron should be reckoned as a Z atom, but ferrous iron should be considered as an X atom. In table 3, sec- tion J, iron is taken to be ferrous; to regard it as ferric would increase the value of Z, but decrease X and Y. Sample computation on the most iron-rich sample was made on the basis of the assumption of ferric iron and produced negligibly different final results. With a few exceptions that are obvious from table 3, section J, I have accepted only those analyses that, in addition to having good total oxide sums, satisfy the three conditions Z=4.00i0.03, X=1.00i0.04, and Y=0.0i0.07. The quality of the data, by these tests, is comparable to that of data included in the compilation of Deer, Howie, and Zussman (1963) for plagioclases, as shown by the following average data, standard deviations, and ranges (all calculated to eight oxygens) : Deer, Howie, and This re ort Atom (19651152133120) (table 3, segtion J) Z ___________ 3993:0013 3999:0009 Range in Z __ 3.958—4.020 3.979—4.017 X ___________ 099610.014 1.017:0.023 Range in X __ O.968—1.030 0.962—1.063 Y ___________ —5.8><10‘3:0.0‘37 +3.9X10‘3i0.050 Range in Y __ —0.0‘81—+0.168 —0.071-—+0.082 Number of analyses ___ 87 51 A matter of considerable interest is whether the sodium-rich plagioclase in rocks below the staurolite zone shows the peristerite miscibility gap. Crawford (1966) demonstrated that certain schists from east— ern Vermont and from New Zealand show this gap, which decreases as metamorphic grade increases until it disappears in rocks of the staurolite zone. Crawford’s data show that at approximately the garnet isograd, plagioclase in the bulk composition range Ab.,,-,—Ab80 does not exist as a single phase; instead two plagioclases are present side by side. The miscibility gap shOWs a marked asymmetry to- ward the sodium end. Orville (1974) suggested that instead of being the result of a two—phase solvus, the peristerite gap may be the result of a transition loop between low albite and high plagioclase that has the maximum temperature point anchored at the pure albite composition rather than at some intermediate composition. Nord and others (1978) examined several speci- mens of plagioclase from the report area by detailed MINERALOGY AND MINERAL ASSEMBLAGES 21 microprobe study and also examined sample 3—3 by means of transmission electron microscopy. The plagioclase shows concentric zoning; the interme- diate zone shows spinodal decomposition into peris- terite. The peristerite structure has characteristic dimensions of a few hundred angstroms and is found only in regions where the microprobe analysis (beam size about 5 ,ma) shows the composition to be in the An5—An20 range. The texture of the exsolution and its restriction to zones of suitable composition suggest that the zones originally crystallized as a homoge- neous phase. Because the rock probably never at- tained a grade as high as that of the garnet meta- morphic zone, crystallization of a homogeneous oligoclase instead of a two-phase peristeritic mixture was likely metastable, and the reason for this met- astable single—phase crystallization is at present unknown. Other plagioclase samples from below the garnet and the chloritoid zones studied by Nord and others (1978) show a complex range of zoning patterns; the reader is referred to their paper for details. ILMENITE Ilmenite is the most common and readily identified opaque accessory mineral from the rocks of the area. It is characteristically platy, a fact that greatly aids in identification. In low-grade pelites, it is commonly sandwiched between chlorite plates (fig. 4), and lo- cally it is sandwiched between white mica plates. Microprobe analyses of ilmenite are given in table 3, section K. Ilmenite grains enclosed in garnet cores of sample 356-1 contain more manganese than those in the matrix as a result of the exchange equilibrium between garnet and ilmenite. The results are shown in figure 12. Least squares analysis of the data is unsatisfactory because the points fall into two tight clusters. However, the line in figure 12, drawn by use of coeval fitting (Zen and Albee, 1964), indicates that, in atomic proportions, Mn(Ga) = —3.4><10*3+ 16.6 Mn(Ilm), r2 (correlation coefficient) =0.97 on the basis of a 12-oxygen formula for garnet and a 3-oxygen formula for ilmenite. MAGNETITE Magnetite is found in some samples, as indicated in table 1. Microprobe data for one sample are given in table 3, section L. These data and those for ilmen- ite show that neither mineral is a host for zinc, and thus the source of zinc in staurolite remains a problem. 0.4 0.3 — — Mn IN GARNET, ATOMS PER 12 OXYGEN o N l l 0 I l O 0.01 0.02 0.03 Mn IN ILMENITE,ATOMS PER 3 OXYGEN FIGURE 12.—Manganese content of coexisting garnet and ilmenite (as inclusions in garnet) in sample 356—1. The high-manganese points were taken from the core of a large garnet; one of the low-manganese points was from the rim of the same crystal, and the other points were from the rim of another small garnet crystal. TOURMALINE Tourmaline is a common accessory mineral in the pelitic rocks, especially of the Everett Formation. These crystals are as much as 0.1 mm across, are commonly strikingly zoned, and have a honey-yellow core and smoky blue-gray mantle. The cores have rounded outlines and presumably were detrital heavy-mineral grains in the sediment. However, the mantles are metamorphic, because they have ir- regular, jagged to euhedral outlines and also because larger grains actually contain preexisting foliation as dusty trains of inclusions (fig. 13). QUARTZ Quartz is ubiquitous in the assemblages studied (table 1) and is the only polymorph of silica found. All mineral assemblages are assumed to have equili- brated with it. In low-grade clastic rocks, some quartz-rich layers show mark-ed cataclastic texture. In higher grade rocks, on the other hand, such tex- tures are not found, and quartz is present as discrete grains; in quartzite beds, it commonly assumes poly- 22 METAMORPHIC MINERAL ASSEMBLAGES, A, Zoned tourmaline characteristic of tourmaline found in the area; a light-yellow core having rounded (detrital?) outline is mantled by a dark-bluish-gray overgrowth; sample 509—1 (planeapolarized lighrt). B, A mantle of tourmaline showing inclusion of a preexisting metamorphic foliation; sample 102—2 (plane-polarized light). FIGURE 13.—Photomic-rographs of tourmaline. gonal contact relations, indicating the approach to textural equilibrium. Veins and stringers of quartz are common in the higher grade pelites; these are interpreted as having been secreted out of the rock rather than having been introduced. In some members of the Stockbridge For- mation, vein quartz and nodular quartz are common, and these form nuclei for the growth of tremolite. Such vein and nodular quartz is never found in the TACONIC ALLOCHTHON, MASS., CONN., N.Y. low-grade stratigraphically equivalent beds. How- ever, these lower grade beds do contain chert nodules and even chert beds as much as half a meter thick; the quartz in the higher grades is presumed to be metamorphosed chert. MODEL MINERAL COMPOSITIONS To facilitate calculations of balanced reactions in the subsequent sections of this report, reasonable model mineral compositions must be assumed where actual data have not been determined or where uni- variant react-ions must be hypothesized on the basis of divariant reactant and product assemblages. The following sections include the model mineral compo- sitions used. These compositions are based on the microprobe data of table 3. Except for the iron— magnesium proportions for the ferromagnesian phases and the potassium-sodium proportions of white mica and alkali feldspar, the compositions probably are good depictions of the actual phases. Epidote and hornblende contain ferric iron; iron in the model compositions of the other minerals is ferrous. STAUROLITE staurolite composition H (Fe,Mg) 2A1,,Si4024. Iron is variable in the range 1.75—1.85; a useful ap- proximation is FeLsSMg‘ou. The model used is CHLORITOID The model chloritoid composition is H2 (Fe,Mg) A12Si07. A useful approximation is Fe0_86Mgo,14, both $0.02. GARNET The model garnet composition used is (Fe,Mg,Ca,Mn) 3AIZSi3012. The divalent cation proportions range widely: Cal- cium from 0.2 to 0.7, and iron from 2.0 to 2.7. The spessartitic component is subtracted out completely in the calculations. A useful approximation of the Fe/(Fe+Mg) ratio is 0.95. CHLORITE The model chlorite composition used is H8 (Fe,Mg) 4,5A138i25018. This formula differs from the formula of Pauling (1930) and the modified formula suggested by Thompson and Norton (1968) but seems to accord A MODEL MULTISYSTEM 23 much better with the chemical analyses of metamor- phic chlorites from pelitic rocks (Zen, 1960) and the present set of microprobe data. Most of the chlorites have a Fe/Mg ratio of 2.5/2, which is a useful ap- proximation. The proportion may deviate from that value, and, where available, actual values are used in the calculations. BIOTITE The model biotite composition used is H2K0.9 (Fe,Mg) 2.56A11.77Si2.67012- This formula, based on the microprobe data, balances the total cation charge (24) as well as the total num- ber of octahedral and tetrahedral sites occupied (7). The Fe/ Mg ratio is generally about 1.5/1.1 but varies a little (fig. 3). MUSCOVITE The mod-e1 muscovite composition is H,KA1,Si.0.,. The microprobe data show this ideal formula to be a good approximation of actual muscovite, the main deviations being the phengitic substitution in a few low-grade rocks (fig. 2) and the Na=K substitution. The paragonitic component, however, is minor. EPIDOTE The model epidote composition used is HCa2(A12Fe3+)Si3013. This is a good approximation of the actual micro- probe analyses. The ferric iron substitution for alu- minum is nearly constant at one atom per formula as given. HORNBLENDE The hornblende from the area is uniformly high in the tschermakitic component (Doolan and others, 1978). The calculations involving hornblende used the average analyses for the appropriate rocks, slightly simplified (analyses of sample 161—1 are re- ported in table 3, section H; analyses of 102—1 and 289-2 were reported by Doolan and others, 1978) : sample 161—1: HzNa0M4lca163Fe::1Mgog1Fe:1 9 A1,..Si,.,.0.. ' Sample 102—1: HzNao..,,Ca,_“FezzfgzMgL02121324:5 A13.67Si6.02024 Sample 289—2: ,HzNa(,,39C'a1,79Fef:5MngFef):8 A13.3ZSi6.10024 PLAGIOCLASE Formulas of the end-member components are used for plagioclase. KYANITE The ideal formula of kyanite, A12S105, is used. A MODEL MULTISYSTEM Most of the mineral assemblages observed involve various combinations of nine principal minerals: garnet, staurolite, chloritoid, chlorite, biotite, mus- covite, plagioclase, epidote, and quartz. Other ob- served minerals are hornblende, cummingtonite, i1- m-enite, magnetite, paragonite, kyanite, potassic feldspar, and tourmaline. However, if the assem- blages restricted to the nine principal minerals are explained, the presence of additional minerals can be more readily explained by an extension of the theory. If the An end member of plagioclase alone is con- sidered, and if the other minerals are confined to those components given by the model compositions, then the system can be defined by the eight—compo- nent system: SiOz, A1203, Fe, MgO, CaO, K20, H20, and 02. Because quartz is ubiquitous and if both H20 and oxygen are boundary-value components,4 the minerals are reduced to a simple quinary multi- system of one negative degree of freedom. Such a multisystem has eight invariant points which may be labeled by the missing phases as (Ga), (St), (Cd), (Ch), (Bt), (Mu), (Pg), and (Ep)~ These points are connected by univariant lines bearing symbols of two missing phases, the enclosed divari— ant areas bear triple labels, and so on. The system is degenerate because muscovite and biotite must appear in pairs on opposite sides of a reaction and because garnet, epidote, and plagioclase (anorthite) are the only calcium-bearing phases, so that the three invariant points (Ga), (Ep) and (Pg) lie on a smooth univariant loop. Mu and Bt are not directly connected but are indirectly connected through one of the other invariant points (Zen, 1974). As a result of the degeneracy, only 19 dis- tinct univariant lines are in the multisystem instead of the full number of 8!/6!2!=28 lines. All the points except (Mu) and (Bt) have seven univariant lines emanating from them; the points (Mu) and (Bt) have only six emanating lines. ‘The validity of the assumption is, of course, arguable (see Rumble, 1974). For the volatile H2O, I prefer to retain this assumption and use other nonvolatile components such as 0110 to explain the observed assem- blages, unless it is necessary to do otherwise. 24 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. St—Bt-O- Cd-Ch-Mu FIGURE 14.—Closed net for the quinary multisystem Algos—FeO—MgO—CaO—K2O for the phases epidote—garnet-plagioclase- chlorite-chloritoid-staurolite-biotite-muscovite in the presence of quartz and having H20 and 02 as boundary-value com- ponents. Abbreviations: Ep, epidote; Ga, garnet; Pg, plagioclase; Ch, chlorite; Cd, chloritoid; St, staurolite; Bt, biotite; Mu, muscovrite; Q, quartz; W, water; and 0x, oxygen. The model compositions of the text were used to balance the re- actions. Pseudodivariant fields are labeled with italic letters for easy reference. The complete closed net for the multisystem is I these define the entire net; all other reactions are shown in figure 14. The figure involves also 15 in- 1 linear combinations of these three. different crossings. The topology of the net is deter- i The closed net has a stable part and a metastable mined by balancing the 19 univariant reactions. ! part, corresponding to the upper and lower parts of Only three distinct reactions are independent, and | the representation polyhedron (Zen, 1966). Deter— A MODEL MULTISYSTEM 25 min-ation of which set of points constitutes the stable part and which constitutes the metastable part is straightforward if, for a given choice of intensive plotting variables, the values for the con- j ugate extensive variables are known. For example, if we choose the chemical potential of H20 and total pressure to be the plotting intensive variables (isothermal and iso-,uo2 sections), the conjugate extensive variables are the number of moles of H20 released for a given reaction and the change of vol— ume of the solid phases for the same reaction. The former is given «directly by the balanced univariant reactions; the latter is obtainable from tabulated molar volume data for minerals (for example, Robie and others, 1967). The molar volumes I used take into account the fact that the model compositions are generally those of solid solutions; the correc- tions are simply linear combinations of end-member volumes because more refined data are not available and constitute second-order corrections. The data used are given in table 6, as are the bases for their derivation. Knowledge of the molar volumes and of the amount of H20 released in the reactions then permits cal- culation of the slope-s of the univariant line-s in Ps—[LW (pressure on the solid—chemical potential of H20 ) space and also permits the unambiguous ori- entations of the Schreinemakers bundles (Zen, 1974). These bundles consist of invariant points and associated univariant lines and belong to one of two groups depending on their mutual consistency. The closed areas of each partial net constructed from the consistent bundles contain phase assemblages unique to each partial net, so that comparison with observed mineral assemblages decides which is the stable and which is the metastable net (Zen, 1974). By application of this approach, the eight invari- ant points (fig. 14) are placed into two groups: (Mu), (Ch), (Pg), and (Ga) in one group and (St), (Rt), (Ep), and (Cd) in the other.5 The 5For the purpose of this construction, I used for chloritoid the com- position appropriate to the analyzed sample from the lowest grade rock containing this mineral, namely 140—2, having a (Fe+Mn)/Mg atomic proportion of 0.88/0.12 (table 4) instead of the mean value of 0.86/0.14. Use of this chloritoid composition enables us to include the invariant point (St) on figure 15 Without in any way changing the topology of the grid at the other invariant points and with but insignificant changes in the values of the slopes around these other points. Inclusion of (St) permits the study of low-grade reactions and is thus desirable. The drastic effect of a small compositional difference of chloritoid on the net is more apparent than real and is the result of the facts (1) that the uni- variant line (Ep, St) is nearly independent of pressure and (2) that a slight change in the stoichiometric coeflicients of the reaction changes the sign of the slope. Note that even though figure 15 uses pressure and the chemical potential of H20 as the independent variables, the diagram can be qualitatively extended to P-T (pressure—temperature) sections be- cause the main effect of temperature increase is dehydration, precisely the same as that of isothermal decrease of the activity of H20. closed areas are defined by the labels of the invari- ant points of each group. The four-point partially closed net consisting of (St) (Ep) (Cd) (Bt) has these possible unique assemblages: 1. (Ep,Bt,Cd,); 2. (Ep,Bt,St); 3. (Ep,Cd,St) ; and 4. (St,Bt,Cd). The other four-point partially closed net has the unique assemblages: 5. (Mu,Ch,Pg); 6. (Mu,Ch,Ga); 7. (Mu,Pg,Ga) ; and 8. (Ch,Pg,Ga). Assemblages 5, 6, and 7 can be dismissed because they cannot coexist with muscovite which is ob- served in every sample. Assemblage 8 can be dis- missed because all observed chloritoid-bearing as- semblages have chlorite. On the other hand, assem- blages 1 through 4 are all observed (for example, sample 355-1 contains assemblage 1, 16—1 contains assemblage 2, 370—2 contains 3, and 62—1 contains 4; see table 1). Therefore, I conclude that this par- tially closed net is the correct one for the area; this is shown in figure 15. The net contains three of the fifteen possible indifferent crossings; the others are excluded either because they are unique to the other net or because the numerical values of the slopes on the P,— ,uW projection exclude the intersection of the lines. The net was constructed by assuming that oxy— gen behaved as a boundary value component. If oxygen is an initial value component so that the value of its chemical potential is determined by the buffered assemblage itself, then the multisystem has one more independent component without add- ing a phase. Because one more phase may be added without changing the variance, epidote (of variable composition) or another mineral containing ferric iron (magnetite, hematite) may be found with the assemblages. Alternately, one or more of the other minerals may show variable amounts of ferric iron. The nature of the mineral assemblages, to be dis- cussed in the following sections, shows that this possible interpretation is compatible with the ob- servations but is hardly necessary. It cannot be overemphasized, as the example given demonstrates, that the assignment of invariant points and indifferent crossings to the two sets depends on numerical values of the slopes, which, given the nature of the Schreinemakers bundles, decide the orientations of the bundles in the chosen intensive-variable space. These slopes represent differences of large numbers, some having large uncertainties. Therefore, if molar volumes of the minerals are refined and if different mineral compositions are assumed, the results could be changed. 26 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. (Ch) (Ga) (Mu) M (Ch) N \ (Mu) K (Ep) 0 (MU) L (C1) A B\ (St) (Ga) 3 3. (Pg) . J ‘(Cffl (Bt) I D H 3 Relative § scales G (Ga) 8 E F a: (P9) 3 (Pg) ' .3. (Ch) (Ga) (99) 1bar Ps—>’ FIGURE 15.—A part of the multtsystematic net of figure 14 containing the four invariant points (St), (Ep), (Bit), and (Cd). This partial net predicts assemblages in conformity with those observed. The model compositions of the minerals (except chloritoid, see footnote, p. 25 for discussion) and the molar volumes given in table 6 were used in orientmg the diagram and determining the scales. Labels With italic letters of pseudodivariant fields are same as in figure 14. PROBLEMATIC PHASE RELATIONS The mineral assemblages in the study area that are summarized in table 1 present a few interesting problems. In this part of the paper, these problems rather than the details of each mineral assemblage are discussed. An understanding of the solutions to the problems will greatly aid in clarifying the relationships among the mineral assemblages and will provide a model describing the progressive evolution of these assemblages. FIRST APPEARANCE OF CHLORITOID The reaction leading to the first appearance of chloritoid is not well understood. Some of the pos- sible reactions have been reviewed by Frey (1969) and Zen and Thompson (1974). Thompson (1972) suggested the reaction chlorite+hematite=chlori- toid+magnetite, thus relating the appearance of chloritoid to the change of the rock color from purple (chlorite+hematite) to green, as purple rocks seldom lack chlorite. In western Vermont (Zen, 1960), as well as in the study area, however, many green pelitic rocks, some containing magne- tite and some not, do not contain chloritoid; others have chloritoid but no magnetite. In western Ver- mont, rocks stratigraphically equivalent at lower grades to these green rocks are purple. Therefore, the coupling of the color change and appearance of chloritoid cannot be a general phenomenon. Frey (1969, p. 115) suggested that the reaction in the Keuper Formation of central Switzerland that led to the formation of chloritoid was between an aluminou-s chlorite and titaniferous hematite to PROBLEMATIC PHASE RELATIONS 27 form chloritoid, rutile, and a less alumin‘ous chlorite. The microprobe data of samples from the study area and previously published chemical analyses of chlo- rite (Zen, 1960) do not show a decrease of alumi- num content of chlorite upon the appearance of chloritoid; no indication exists that titanium par- ticipated in the formation of chloritoid, and no rutile is found. Therefore, the reaction suggested previously (Zen, 1960) may be valid: chlorite+ paragonite=chloritoid+albite. If H20 is assumed to be a boundary-value component, this reaction in the presence of quartz involves the components sodium, iron, magnesium, and aluminum. All four phases (and quartz) could coexist under divariant conditions. If the plagioclase contains calcium and no other calcium mineral is present, the system is even trivariant, and the four-phase assemblage may be widely expected. In western Vermont, the chlo- ritoid-chlorite assemblage is found only with parag- onite, not plagioclase (Zen, 1960). In the present study area, both paragonite and plagioclase, to- gether or singly, are found with the chlorite-chlo- ritoid pair (table 1). This coexistence could be caused by the addition of the calcium component in plagioclase in the Massachusetts rocks but could mean that these rocks are more anhydrous (higher grade) than those of Vermont. THE CHEMOGRAPHY OF GARNET AND THE BIOTITE-CHLORITOID-GARNET-CHLORITE RELATIONS In the study area, biotite and chloritoid are com- monly found together. They generally form stout porphyroblastic laths in intimate association with— out sign of reaction (fig. 16) or indication that they formed during different episodes of metamorphism. The presumption must be that these two minerals formed together in a state of at least local equilib- rium. Albee (1972) and other workers have dis- cussed possible reasons why this biotite-chloritoid assemblage is found instead of the common assem- blage of almandine—rich garnet, chlorite, and musco- vite in pelitic rocks of the garnet zone and the lower grade part of the staurolite zone. Albee (1972) suggested that, in the presence of quartz and muscovite, the chlorite-garnet join is broken at higher temperature to yield the chloritoid— biotite pair in rocks having the appropriate bulk ratio of magnesium to iron. The mineral assemblages from the area provide data to test this hypothesis. Almandine-rich garnet is present in 117 assem- blages of the rocks examined pe‘trographically in this study (table 1). Without exception, the rocks containing garnet are pelitic. All but nine of these garnetiferous rocks also carry chlorite. Excluding rocks that contain chloritoid only as inclusions within garnet, which may not have formed in equi- librium with the minerals outside of the garnet, 14 rocks contain the four-phase association garnet- chlorite—chloritoid-biotite (plus quartz, muscovite, and plagioclase). Only two assemblages (466—1 and 466—2) show the chloritoid-biotite-chlorite-musco- Vite association without garnet; these are found at the garnet-staurolite zone boundary. To a good first approximation, the biotite-chloritoid association is coextensive with the garnet-chlorite-muscovite as- sociation and is not complementary to it, as it would be if a reactive relation always existed between them (both Albee’s sample 242—c from “Patterson Kno ” in his 1972 study and my sample 103—1 are from the same outcrop; on the US Geological Sur- vey topographic map of Bashbish Falls Quadrangle (1958), it is a knob identified by the triangulation station “Patterson”). Of the 14 assemblages of chloritoid-chlorite-bio- tite-garnet sampled in the area, four have been studied by use of the electron microprobe, whereby each of the phases was analyzed. The results are presented in table 3, but for convenience, these are gathered together as average values of cations per formula for each mineral from each rock and are presented in table 7. In addition, one sample (339—1) for which the chloritoid was found only as inclu- sions in the cores of almandine crystals was also analyzed, and the results are given in table 7. Re- sults for sample 466—1, which contains the chlori- toid-biotite pair without garnet, are also given in table 7 for comparison. From the data, one could next calculate the bal- anced chemical reactions involving the transition from the garnet-chlorite assemblage to the chlori- toid-biotite assemblage. The calculations are analo- gous to those used for the model multisystem, except that the actual iron—magnesium values for garnet and biotite (table 6) were used and the system was assumed to be lime free. The chlorite formula was further tested by using two different versions. One formula suggested by Thompson (1957) and Thompson and Norton (1968) may be written as (Fe’Mg) 4.67A12.67Si2,s7010(OH)8; the second, the model formula (Fe,Mg)4.5Algsi2.5010(0H)8, makes the (Fe,Mg)/Al ratio of chlorite equal to that of almandine-pyrope garnet. Balanced chemical reac- tions using all five garnet-bearing assemblages of table 7 lead to these conclusions: 28 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. FIGURE 16.—Photomicrographs of chloritoid (Cd)-biotite (Bt) assemblages. A, Sample 338—1, with chlorite (Ch), garnet (Ga), and muscovite (Mu). B, Sample 234—1, with chlorite (Ch), staurolite (St), muscovite (Mu), plagioclase (Pg), and ilmenite (Ilm). C, Sample 103—1A, with garnet. D, Sample 509—1A, with garnet (Ga) and chlorite (Ch). All, plane-polar- ized light. 1. The proportions of the phases participating in 3. A small amount of quartz (about 2 moles per each reaction are nearly independent of the choice of chlorite formula, although for stoi- chiometric reasons, the more aluminous for- mula leads to a constant relation between the amount of biotite and the amount of chloritoid for all five assemblages. 2. The stoichiometric coefficients of each phase par- ticipating in the reaction are nearly independ- ent of the Fe/ Mg ratios of the phases. mole of chlorite) is needed on the biotite+ chloritoid side to balance all five reactions. 4. A small amount of H20 is released (about two- thirds mole per mole of chlorite) upon the formation of biotite+chloritoid. This release of H20 confirms Albee’s (1972) conclusion that the biotiteechloritoid assemblage is of higher grade than the garnet-chlo‘rite assem- blage, everything else being equal. The amount of H20 released, however, is small. PROBLEMATIC PHASE RELATIONS 29 In the calculations, the manganese and equivalent amounts of aluminum and silicon in the garnet have been subtracted. This may seem a bad approxima- tion because the manganese content, in relative divalent cation proportions, is an order of magni- tude greater in garnet than in the other phases. However, the rims of the garnets are considerably less manganese rich than are the cores, and the data in table 7 are averaged without regard to the posi- tion of the analyzed spot in the garnet. I presume that the phases that would have reacted initially would have had compositions corresponding to those of the rims. The data in table 7 for sample 466—1 show that the chloritoid, chlorite, and biotite of this assem- blage are not discernibly different from those of the other assemblages. The sample came from near the boundary of the staurolite-garnet zones and is approximately isogradic with specimens 103—1 and 103—2. Pelitic rocks on both sides of the isograd (for example, samples 448—1, 366—2, 463—1, and 468—1) carry garnet (:staurolite). Thus, the lack of garnet in samples 466—1 and 466—2 is probably due to the bulk composition of the rocks rather than to metamorphic grade. In interpreting the data of table 7, one must re- member that the 16 biotite-chloritoid assemblages are found evenly distributed in the garnet zone and in the lower grade part of the staurolite zone (table 1). Except for samples 466—1 and 466—2, all these as- semblages contain garnet and chlorite, and the min- erals show no evidence of disequilibrium and reac- tion, indicating that these “alternative” assemblages are in fact equilibrium assemblages. Moreover, be- cause so many biotite-chloritoid-garnet-chlorite as- semblages were found to be widely distributed in the study area, the observed relations probably are not the result of the samples having been obtained by chance on an univariant reaction surface. If the bio- tite-chloritoid-chlorite-garnet-muscoviteassemblages are divariant or have even higher variancy, clearly more components than K20, FeO, MgO, and A1203 (+Si02 and H20) must be considered in a model multisystem. Possible candidates are Fe203, Ti02, MnO, CaO, and Na20. In the phase rule sense, we may rule out NaZO because it is an essential component of plagioclase, a ubiquitous phase in these assemblages. TiO2 is only a trace component in all the phases, and ilmenite is generally present in the assemblages. The micro- probe data cannot show the amount of ferric iron in the phases, and the rocks do not carry hematite and rarely carry magnetite, so this component can- not be definitely excluded. The stoichiometric cal- culations of chloritoid and garnet analyses indicate that ferric iron cannot be important in these min- erals, but it may be important for biotite and espe- cially chlorite. Although optically the chlorites in these assemblages do not appear to be oxychlorites (Chatterejee, 1966), ferric iron in fully hydrated chlorite cannot be distinguished optically. Hey (1954) proposed a regression equation for ferric- iron content versus refractive index; however, the standard deviation of the results on which the re- gression was based is about equal to Hey’s reported total range of iron (0 to about 3 atoms per 18 oxy- gen), so the use of the equation is not practical. (See equation 1, Deer, Howie, and Zussman, 1962, p. 152.) The manganese and (or) calcium content of gar- net potentially could explain the assemblages. The microprobe and wetehemical analyses of garnets (tables 3, section F, and 4) show that both compo- nents are almost never absent from garnet. Manga- nese is strongly concentrated toward the cores of the garnets; calcium content tend-s to be uniform in garnet of a given rock, although it changes sen- sitively in different rocks and different mineral as- sociations and can occupy as many as one-fourth of the total divalent cation positions for hornblende- bearing rocks. It is not possible to decide whether MnO or CaO is responsible for the assemblages. However, because of the factors just mentioned, in- cluding the different distribution of manganese and calcium in garnet, biotite-chloritoid and garnet- chlorite-muscovite are considered not to be alterna- tive assemblages but to represent different chemo- graphic regions, the latter association being in rocks containing more calcium than can be absorbed in the plagioclase. In pelitic rocks containing the gar- net-chlorite-muscovite assemblage, garnet is not a phase truly belonging to the AFM projection of Thompson (1957) but is interior to a ACFM tetra- hedron.6 The conclusions discussed form the principal basis for the choice of the model mineral compositions in the multisystem shown in figure 14. As shown in figure 14, the reaction proposed by Albee (1972), now taking into account the effect of calcium in garnet, becomes the curve (Ep, St) ; the chloritoid- biotite-anorthitic plagioclase assemblage is the 6Table 3, section F, shows in fact that only one rock contains nearly calcium-free garnet. This is sample 487—2, whose garnet contains the normal allotment of calcium in the core but is nearly free of calcium toward the rim. The assemblage is garnet-chlorite-cummingtonite-magnetite-biotite- quartz and has only traces of muscovite and no plagioclase. 30 METAMORPHIC MINERAL ASSEMBLAGES, ‘TACONIC ALLOCHTHON, MASS., CONN., N.Y. drier and denser equivalent of the garnet-chlorite- muscovite assemblage. However, the plagioclase in assemblages from the study area is a solid solution rich in the Ab component, and the assemblages do not have a sec- ond sodium-bearing phase, except the solid solution of sodium in muscovite (none of the paragonite- bearing rocks have the assemblage being discussed). Therefore, the assemblage garnet—chlorite-biotite- chloritoid-plagioclase-muscovite-quartz occupies a divariant field (fig. 15) and does not lie merely along a univariant line. The fields in which the as- semblage is stable are below and to the right of the lines (Pg, Ep) and (Ep, St). Sample 466—1, containing the assemblage chlo- ritoid-biotite-muscovite-chlorite-plagioclase-quartz, without garnet, is found close to rocks containing both garnet and staurolite. The absence of garnet is interpreted to reflect composition of the rock rather than conditions of metamorphism. Likely divariant fields where the assemblage could have formed are fields U and R in figure 15. Crawford’s (1974) discussion of the buffer rela- tion between a calcium—bearing garnet and a cal- cium—bearing plagioclase in pelitic rocks was mod- eled on the experimental work of Boettcher (1970) on the CaO—A1203-Si02-H20 system. Reactions sug- gested by Crawford are low-calcium garnet+ calcic plagioclase = high-calcium garnet + sodic plagioclase + aluminum silicate + quartz, low-calcium garnet + calcic plagioclase + biotite = high-calcium garnet + sodic plagioclase + muscovite +chlorite + quartz, and low~calcium garnet + calcic plagioclase + H20 =high-calcium garnet+~sodic plagioclase + staurolite + quartz. None of these reactions as such can be applied to our rocks. The possibility of a buffer reaction can- not be adequately represented by the proposed mul- tisystem, which assumes fixed phase compositions. A likely reaction might be low-calcium garnet+calcic plagioclase+ biotite + chloritoid + H20 + quartz :high-calcium garnet+ sodic plagioclase + chlorite + muscovite. This reaction is analogous to the limiting reaction depicted by the line (St, Ep) on figure 15. In a study of garnet-staurolite-biotite assem- blages from Nova Scotia, Phinney (1963) suggested that the CaO component may significantly affect the nature and compositions of the assemblages. Phin- ney’s argument, based on the behavior of tielines of coexistent phases, was later shown to be non- compelling (Greenwood and others, 1964). How- ever, Phinney’s initial conclusion may remain valid, and his assemblages may provide a higher grade analogy with the phase relations observed in my area. Rumble (1974) studied some pelitic rocks in west- ern New Hampshire that carry assemblages includ- ing garnet, chloritoid, chlorite, kyanite, and stau- rolite. He found apparent crossings of tielines as well as too many phases in the AFM projection of Thompson (1957). Rumble was able to unscramble the tielines by projecting the assemblage-s through muscovite and kyanite or an iron end member of staurolite onto the HZO-FeO-MgO plane. He there- fore suggested that the assumption of boundary- value component status for H20 is invalid and that strong gradients in the chemical potential of H20 existed during metamorphism. This suggestion may be correct, but the inclusion of another component is bound to reduce the problem of excess phases. Rumble’s projection through muscovite and kyanite sidesteps the problem of crossing tielines because the problematic assemblages are not visible from the kyanite projection point. Projection from an idealized iron-staurolite may or may not be valid because staurolite is not fixed in composition and assemblages including non-end-member composi- tions of this mineral are those that most commonly appear to violate the phase rule. Thus, although Rumble’s suggestion is stimulating, I have chosen not to utilize it in explaining my assemblages be- cause his rationale is not necessary. CHLORITOID-STAUROLITE RELATIONS In recent years, many experimental and observa- tional studies have been made on the phase relations of coexisting staurolite and chloritoid in metamor- phic rocks. Ganguly (1968, 1969, 1972) studied the experimental phase equilibrium relations of both minerals. Grieve and Fawcett (1974) studied the stability relations of chloritoid and revised the ear— lier results of Halferdahl (1961). Richardson (1968) and Hsu (1968) worked on the upper stability of staurolite. Hos-chek (1967, 1969) studied the stabil- ity of staurolite in multicomponent systems approx- imating the compositions of pelitic rocks. These are but some important studies among many. As use of the electron microprobe has become Widespread, chemical data on coexisting staurolite PROBLEMATIC PHASE RELATIONS 31 and chloritoid also have become increasingly avail- able. These data, coupled with the observation that chloritoid and staurolite are commonly found to- gether without obvious evidence of one mineral growing at the expense of the other, led to the ap- preciation of the fact that these minerals form co- existing pairs in rocks of suitable composition. The compositions of these pairs, as well as their reaction relations, are of considerable interest. 0f the mineral assemblages studied, 46 contain staurolite. Among these, ten also contain chloritoid as a groundmass phase. The compositions of five of these coexisting staurolite and chloritoid pairs have been studied by use of the microprobe and are given in table 3, sections D and E. Sample 339—1 (table 3, sections D and E', and table 7) contains staurolite in the groundmass, but the chloritoid is present as inclusions in the core parts of the garnet. The chem- ical data are useful for comparison. Albee (1965, 1972) studied the chemographic re- lations of chloritoid and staurolite and made good use of the graphical technique of Thompson (1957). Hoschek (1969), in part on the basis of his experi- mental phase equilibrium studies, proposed two pos- sible reactions involving chloritoid and leading to the appearance of staurolite: chloritoid + aluminum silicate = staurolite+ quartz + H20, chloritoid + quartz = staurolite + garnet + H20. Schreyer and Chinner (1966) made microprobe studies of the Fe/ Mg ratios of some coexisting chloritoid and staurolite from Big Rock, N. M., and concluded that chloritoid is slightly more side- rophile than the coexisting staurolite. This relation, if true, would seem to suggest that Hoschek’s second reaction for the derivation of staurolite is more probable. However, Rumble (1970, 1974) , also using the microprobe, found that coexisting staurolite and chloritoid from a single formation from western New Hampshire have identical Fe/ Mg ratio-s in the presence of garnet; however, in assemblages with- out garnet, the chloritoid is more magnesium rich than is the coexisting staurolite. Because the garnet that coexists with chloritoid and staurolite shows the highest Fe/Mg ratio of the three minerals, Rumble (1970) advocated a reaction of the type quartz + chloritoid + aluminum silicate = garnet + staurolite + H20. In fact, however, the chemographic relations given by Rumble show that the components of garnet+ staurolite and chloritoid+aluminum silicate are mutually indifferent in the AFM projection. Fox (1971), Albee (1972), and Fed’kin (1975, p. 74) produced new microprobe data on coexisting staurolite and chloritoid from diiferent metamorphic areas. Albee showed that for the samples analyzed, staurolite always has a higher Fe/ Mg ratio than the coexisting chloritoid does and that this ratio is even higher in the coexisting garnet. Average Fe/ Mg ratios and standard deviations of these coexisting minerals in my study area were calculated from the data given in table 3, sections D, E, and F, and are presented, as follows (figures in parentheses are the numbers of ratios averaged) : Sample Chloritoid Staurolite Garnet 234—1 _____ 5.3:t0.5(2) 7.7:L-0 9(3) 11.6:1.1 (2) 331—1 _____ 5.7:0.3(8) 7.210.5(6) 11.9:1.0(9) 369—1 _____ 6.4:0.2(4) 8.7:0.3(2) 14.411.9(4) 463—1 _____ 5.7:0.3(6) 6.3:0 4(2) 11.311.5(3) 506—1 _____ 6.410.2(4) 7.5:0 6(3) 12.1:0.4(4) Thus, my data, as well many of Rumble’s new (1974) data, are entirely in accord with Albee’s conclusions. The data indicate that none of the three reactions listed above could account for the first appearance of staurolite because opposite sides of each reaction form compositionally indifferent systems. The contradiction between Albee’s, Fox’s, and my data and those of Schreyer and Chinner remains unexplained. Taking the different chemical data together, one might be tempted to conclude that the relations indicate an extremal type of phase rela- tions between chloritoid and staurolite (see sum- mary by Khlestov, 1972) ; however, the evidence is permissive at best, and such a conclusion seems to be premature. Another possible explanation is that different extents of ferric iron substitution for alu- minum might affect the Fe/Mg ratios, because nearly all the data are derived by the microprobe. Albee’s data (1972) do not readily lend themselves to sorting out possible Fe3+ =A1 substitution. How- ever, on the basis of stoichiometry, the data of table 3 and of Fox (1971) show that these chloritoids are virtually free of ferric iron. If ferric iron is impor- tant in staurolite, then the correct Fe2+/Mg ratio would show staurolite to be less siderophile than indicated by the microprobe data, thus conforming better with the results of Schreyer and Chinner (1966). Staurolit‘e from sample 355—1, already dis- cussed, does not support the idea of significant ferric—iron contribution. The microprobe data also calculate out to a nearly full complement of alu- 32 minum (9 per 24 oxygens). On the basis of these data, as well as those of Fox (1971), ferric iron does not seem to be an important component of staurolite in coexistence with chloritoid, and the relatively higher Fe/Mg ratio in staurolite seems real. The very lowest grade appearance of staurolite (or the very highest grade disappearance of chlori- toid) in pelitic schists must involve a reaction or reactions in which, except for phases in excess, staurolite appears alone on the product side (or chloritoid appears alone on the reactant side). Con- sidering the chemical compositions of coexisting chloritoid, staurolite, and garnet, and ignoring for now the calcium and manganese contents of garnet, the most likely reaction for the lowest grade appear— ance of staurolite in rocks from the study area seems to be of the type chloritoid + garnet + aluminum silicate = staurolite. Using the calcium— and manganese—free model composition of phases, a possible balanced reaction for the above is chloritoid + garnet + 7 pyrophyllite = 2 staurolite + 3 quartz + 7 H20. In this reaction, muscovite cannot be a reactant because otherwise biotite would have to be formed, contrary to hypothesis. The parag-onitic component or paragonite (Pa) as a phase could participate, however; a schematic reaction, using the model com— positions of phases, is 4 quartz + chloritoid + garnet + 7 paragonite = 2 staurolite + 7 albite + 7 H20. Muscovite can appear on the product side, and so a possible reaction is garnet + biotite + aluminum silicate = staurolite + muscovite However, such a reaction would imply the absence of the staurolite—chloritoid join upon the first ap- pearance of the former, Which seems to contradict petrographic observations. It also requires a biotite— aluminum silicate assemblage, which has not been observed, below the staurolite zone. Reaction among biotite, chloritoid, and aluminum silicate is chemically indifferent to staurolite in the AFM projection, as is a reaction involving chlorite in place of an aluminum silicate polymorph; neither reaction can lead to the first appearance of stauro- lite. After staurolite becomes intrinsically stable, its appearance in rocks of suitable composition can re- sult from switching one or more tielines. If stauro- ME TAMORPHIC MINERAL AS SEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. lite formed originally in the chloritoid-garnet- aluminum silicate compositional field as postulated, then the next higher grade reaction may be of the sort chloritoid + aluminum silicate = chlorite + staurolite. (See also discussions by Thompson and Norton, 1968.) Three reactions that can be written using the model compositions are 23.35 pyrophyllite+25.58 chloritoid = 10.54 staurolite + chlorite + 39.65 H20 +74.31 quartz, 23.35 kyanite+25.58 chloritoid = 10.54 staurolite + chlorite + 16.31 H20 +4.27 quartz, and 23.35 paragonite+25.58 chloritoid+ 19.08 quartz =10.54 staurolite+chlorite+23.35 albite +3965 H20. Note that chlorite is the prograde mineral despite its high degree of hydration. These reactions are analogous to that for the staurolite-chlorite isograd proposed by Fox (1971) and may reasonably be supposed to describe the formation of much of the staurolite marking the staurolite zone mapped in figure 1. Because paragonite has been observed in the garnet zone (tables 1 and 2), the reaction in- volving paragonite may be the one that actually took place. The nature of the mineral assemblages observed and the compositions of individual minerals show that the next plausible reaction is as follows, where the stoichiometry is based on the model composi- tions used previously: chlorite + 2.01 muscovite + 4.21 chloritoid = 2.23 biotite+ 1.50 staurolite + 7.23 H20 + 0.78 quartz. This reaction is (Ep, Pg) = (Ep, Ga) of figure 15 (the Fe/ Mg ratios of the phases can change rather extensively without affecting the qualitative nature of the reaction). This reaction belongs strictly to the AFM projection on the basis of considerations already given to the mineral compositions. It con- stitutes a tieline switch above the staurolite-chlorite isograd that breaks up the chloritoid-chlorite com- patibility. Each of the three-phase subsystems de- rived from the above tieline switch can have garnet as another phase to form two sets of two assem- blages each. Before chloritoid—chlorite (—muscovite) I ceases to be stable, the possible assemblages are (1) PROBLEMATIC PHASE RELATIONS 33 garnet—chloritoid—chlorite—biotite and (2) garnet— chloritoid—chlorite—staurolite; afterwards, these are (3) garnet—ch]oritroid—staurolite—biotite and (4) garnet—chlorite—biotite—staurolite. Reference to table 1 shows that of the four assem- blages (all in the presence of quartz, muscovite, and plagioclase), all but assemblage 3 is found in the area. The absence of this assemblage may result from incomplete sampling or from a lack of the necessary restricted bulk composition. The distribution of assemblages 2 and 4 on the isograd map (fig. 1, samples indicated by triangles and squares, respectively) is well ordered and de- fines a second staurolite zone, marking the reaction given above.7 As shown in figure 1 (samples indicated by cir- cles), four assemblages carry all five phases, chlo- ritoid, chlorite, staurolite, biotite, and garnet; these assemblages are probably truly fortuitous findings of pseudounivariant assemblages. Except the assem- blage of sample 331—3, the five-phase assemblages occur between assemblages 2 and 4. Because the chloritoid-staurolite pair is stable for a finite range of rock compositions as well as of external conditions these minerals must show ranges of Fe/Mg ratios. The composition of the chloritoid in equilibrium with the first staurolite will be different from that of the last chloritoid in equilibrium with staurolite. Unfortunately, my data do not indicate clearly whether the chloritoid co- existing with staurolite becomes more magnesian in rocks of higher or lower grade. Sample 339—1 contains chloritoid relicts in garnet only and thus presumably the rock is above the zone in which chloritoid is a stable phase for the active bulk com- position. This chloritoid is also the most magnesian of those described in table 7 but is not significantly so (compare ranges of compositions for samples 331—1, 463—1, and 509—1 for both chloritoid included in garnet and stout crystals in the groundmass, table 3, section D). The least magnesium chloritoids listed in table 3, section D, are from 45—1, 140—2, 338—1, 466—1, and 515—1. Samples 45—1 and 140—2 are of rocks below the garnet zone, and 338—1 and 466—1 are from just below the staurolite zone; sam- ple 515—1, however, is from well in the staurolite zone just before the disappearance of chloritoid, as far as can be judged. 7Figure 15 indicates that the reactiou (Ep,Pl) takes place at a higher grade than does (Ep,Bt), which describes the reaction of chlori- toid with anorthite to form staurolite+garnet+chlorite; however, this fact need not unduly disturb us because in actual rocks the plagioclase is an Ab-rich plagioclase, not anorthite. How does chloritoid finally disappear from the assemblages? I do not have definite information. However, one of these reactions seems plausible: (1) chloritoid+ muscovite = biotite + staurolite + cal- cium-free garnet or (2) chloritoid=staurolite+ chlorite+calcium-free garnet. Both reactions to the right consume quartz but release large amounts of H20. In calcium-bearing rocks, the reaction is either ( 3) chloritoid + calcium-plagioclase + quartz = stau- rolite + garnet + chlorite + sodium-plagioclase ( ( Ep, Bt) of fig. 15) or (4) chloritoid+calcium-plagio- clase = staurolite + chlorite + epidote + quartz +sodium-plagioclase ((Ga, Bt) of fig. 15) ; reaction 4 is followed by (5) chloritoid+e=pidote=staurolite +garnet+chlorite ((Bt, Pg) of fig. 15). All three reactions release H20 to the right. In sample 355—1, chloritoid is abundant in cores of garnet but never in the matrix and could be a record of reactions 3 or 5. After the disappearance of chloritoid from rocks typified by sample 355—1, the system of four phases (chlorite—staurolite—biotite—garnet, for example, sample 356—1) and "five components acquires an extra degree of freedom; each phase may therefore have one compositional variability that is of course correlated among the phases. Depending on the CaO and MnO contents of the effective rock com- position, the apices of the four-phase hypervolume may vary. Moreover, three-phase assemblages, for instance chlorite-staurolite-garnet (sample 355—1) and biotite-staurolite—garnet, can exist under exter- nal conditions similar to those acting on the four- phase assemblage. These three-phase assemblages will occupy hypervolumes in the compositional space and, for arbitrary effective rock compositions, can be realized. This existence of the hypervolumes appar- ently explains why rocks having few of the ACFM phases, such as sample 355—1, are commonly found in the area. KYANITE-BEARING ASSEMBLAGE One kyanite-bearing mineral assemblage has been found in the study area. Sample 655—1—1 is from the southeastern extremity of the area and repre- sents rocks of the highest metamorphic grade. The assemblage is kyanite-staurolite-biotite—garnet-pla- gioclase-quartz-tourmaline-muscovite-ilmenite; pri- mary chlorite is not found in the several slides examined. The assemblage clearly cannot be dis- cussed in terms of the model multisystem. However, in view of the nature of the mineral assemblages just below this grade, discussed in the previous sec- 34 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. tions, a plausible reaction leading to the assemblage may be staurolite + chlorite + muscovite = biotite + kyanite + quartz + H20 This reaction may mark the final disappearance of the staurolite-chlorite pair in the presence of mus- covite. Using the model compositions, the balanced reaction is staurolite + 4.96 chlorite + 8.55 muscovite = 9.5 biotite + 16.38 kyanite + 0.33 quartz + 19.4 H2O. The main geologic interest in the kyanite-bearing assemblage in the area is in the implication of mini- mum rock pressure during metamorphism, as is discussed in another section of this study (p. 42). PHASE RELATIONS INVOLVING EPIDOTE Epidote is found in rocks ranging from below the biotite zone through the lower part of the stau- rolite zone. Together with garnet, hornblende, and plagioclase, it is an important calcium-bearing phase; its relations with the other minerals thus are of interest. The assemblages that include epidote are given in table 1; table 8 summarizes the data and includes information on the metamorphic zones. At low metamorphic grades, the pelitic rocks show very simple mineral assemblages: muscovite, chlorite, a sodium-rich plagioclase, quartz, and i1- menite are common, and a carbonate mineral (cal- cite) and (or) epidote are added phases. The sim- plicity of these prevalent assemblages has been a puzzle, and I suggested (Zen, 1960) that chlorite may be sufficiently variable in composition to ac- count for this assemblage. The microprobe data on sample 1169-1 (table 3, section A; fig. 2) indicate that for this, and presumably for other rocks, the muscovite is phengitic. The variable composition of muscovite, coupled with variable composition of chlorite along both the Fe=Mg and (Fe,Mg) +Si =Al+Al coordinates, could explain the simple as- semblages, though whether this is the principal explanation remains to be proven. A divariant reaction of i11ite+chlorite+calcite to form phengite+epidote seems possible in view of the chemography of the phases. Using the iron end member for chlorite, an illite composition of HZKMAIZ.2Fe3+o,3Si3,5O,2, a phengite composition of HZKAlmFeZ+0.54Si333012, and the model epidote com- position, the reaction is 8.52 illite + chlorite + 6.75 calcite + 0.84 02 + 0.5 quartz = 3.38 epidote + 6.82 phengite + 6.75 C02 +4.02 H20. If hematite participates in the reaction but no redox change is included, then the reaction is chlorite + 34.1 illite + 4.5 hematite + 18 calcite = 9 epidote + 27.3 phengite + 6.32 H20 + 18 CO2 + 4 quartz. Both reactions seem plausible in view of the nature of the assemblages and the associations of epidote and phengite in sample 1169—1. Both reactions in- volve strong evolution of mixed volatile components in the system C-H-O for the reaction to proceed to the right. In fact, of course, Fe2+—Mg substitutions are im- portant, and both reactions are at least divariant, so that no phase needs to disappear in the reaction; rather, both chlorite and the muscovite-phengite solid solutions slide along a univariant composi- tional trend to adjust to the bulk composition for a given set of external variables. In progressive metamorphism, garnet is the next calcium-bearing phase to appear, and the fact that higher grade rocks contain increasing garnet and diminishing epidote suggests a reaction relation between these two minerals. Five of the 19 epidote- bearing pelitic rocks mapped as being above the garnet isograd do not contain garnet, but three of these contain par'agonite (tables 1 and 2). If we assume that paragonite can be projected to the A apex of the ACFM compositional space, then figure 17A shows that two of the possible assemblages are epidote-garnet—chlorite-chloritoid and epidote-chlo- rite-chloritoid-paragonite (all + p1agioclase+ quartz +muscovite). These divisions are compatible with the data in table 1. Figure 17.4 shows that a third four-phase assem- blage, epidote—chlorite—garnet—biotite (+muscovite +plagioclase+quartz) is compatible with the tie- line relations depicted. Because of the chemographic relation of epidote in the ACFM diagram (Whether ferric iron is reckoned as belonging to the F apex or not), the assemblages including this phase do not involve the biotite-chloritoid tieline. Indeed, the assemblage data (table 1) show that although epi- dote is found with either biotite or chloritoid, it is not found With both, despite the common coexistence of these two phases and garnet, as discussed in another section (p. 27). According to these observed relations, the reac- tions involving garnet and epidote may be chloritoid + iron oxide + chlorite + epidote = garnet + oxygen, CONDITIONS OF METAMORPHISM 35 PaA Cd (0.66,0.0.29,0.05) Ep (o.4,o.4,o.2,0) Ga (0.40,0.04,0.53,0.03) Bt (0.25,0,0.44,0.31) FIGURE 17.—The ACFM diagram wherein paragonite is projected to the A apex, for phases in coexistence With epidote. The numbers in parentheses after each phase name refer to the atomic percent of the four apical components in the order of A, C, F, and M. Assemblages are for the presence also of muscovite, plag'ioclase (albite) , and quartz. A, Below the chloritoid-out isograd. B, Above the chloritoid- out isograd. Epidote is reckoned such that ferric iron is plotted as if it were ferrous iron. Phase symbols: Ep, epidote; Pa, paragonite: Ch, chlorite; Cd, chloritoid; Ga, garnet; Bt, biotite; and St, staurolite. chlorite + iron oxide + muscovite + epidote = biotite + garnet+ oxygen, and paragonite + magnetite + epidote + chlorite = garnet + albite. If the detailed compositions of the phases are known, then balanced reactions can be written (Thompson and Norton, 1968; Thompson, 1976). If magnetite is the iron—oxide phase and the silicate phases have model compositions, these reactions would be as follows: 65.6 chloritoid + 43.8 magnetite + chlorite + 7.5 epidote + 133.3 quartz = 74.6 garnet + 73.4 H20 +237 02, chlorite + 2.0 magnetite + 1.4 muscovite + 0.25 epidote + 4.1 quartz = 1.5 biotite + 2.5 garnet + 4.0 H20 + 1.1 02, and 10.5 paragonite + 10.5 magnetite + chlorite + 1.3 epidote + 33.5 quartz = 13.3 garnet + 15.2 H20 + 10.5 albite + 5.6 02. All three reactions involve devolatilization and de- hydration as garnet is formed. The first and the third reactions truly mark the first appearance of garnet in this chemographic system. The second reaction is of the “tieline switch” type (the tieline garnet-biotite versus the plane chlorite-magnetite- epidote). This reaction could be a petrographic marker for the garnet zone for rocks of appropriate compositions. The reactions predict major consump- tion of magnetite; interestingly, the modal content of magnetite drops abruptly at about the lower limit of the garnet zone. Another reaction involving epidote and garnet is chloritoid +epidote + iron oxide + albite + quartz = garnet+ paragonite + H20 + 02; for instance, 2.0 chloritoid + 0.2 epidote + magnetite + 0.3 albite + 3 quartz = 1.8 garnet+ 0.3 paragonite + 0.602 + 1.8 H20. However, the lack of coexistence of paragonite, gar- net, and epidote in the rocks suggests that this reaction did not obtain, although for other assem- blages lacking epidote, the garnet-paragonite asso- ciation is found and shows that the association can be stable for rocks of appropriate, probably low— calcium, compositions. The relatively small amount of epidote in all the observed rocks, compared with the large amount of garnet in many, indicates that the reactions in- volving epidote cannot be the major reactions lead- 36 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. ing to garnet. Other plausible garnet-forming reac- tions are discussed elsewhere in this study. Another caveat is that the plagioclase in all epi- dote-garnet-plagioclase rocks of the area is not anorthite but a solution relatively rich in the Ab component, so the simple model is not strictly appli- cable. As the An component is formed in a given reaction, it is diluted by Ab, and the chemical po- tential of calcium in this phase decrease-s approxi- mately according to Raoult’s Law; thus, the calcium content of garnet must also decrease in general to maintain equilibrium. If we assume the epidote to be fixed in composition (not strictly true, but the change in Al/Fe ratio may not greatly affect the chemical potential of calcium in the phase), then this phase Will remain unchanged in composition but will diminish in amount so the components may dissolve in the garnet and plagioclase to maintain the equality of the chemical potential of calcium in all phases until epidote disappears. After the disappearance of chloritoid by the re- action (Bt, Ep), (Bt, Ga) or (Bt, Pg), as discussed on page 33, the assemblage staurolite-garnet-chlo_ rite-epidote becomes stable together with plagio— clase, muscovite, and quartz (fig. 173) . We have few probe data on the compositions of garnet and plagioclase that coexist with epidote. Al- though the An content of the plagioclase is nearly constant, the calcium content of garnet decidedly is not. Part of the problem may be in analyzing for the zoned garnet; one must be certain that the cor- rect zone (presumably the rim) that equilibrated with plagioclase and epidote is tabulated, an im— possible task. The compositions of these coexisting phases at specified temperatures, pressures, oxygen- buffer conditions, and chemical potentials of H20 need to be calibrated under laboratory conditions; when the coexisting compositions are determined, these phases can be used in the study of natural assemblages. Samples 356—1 and 590—1 contain staurolite- garnet-epidote-chlorite-biotite (plus muscovite, pla- gioclase, and quartz) and are intriguing because in the ACFM projection they contain five phases. One could argue that in this assemblage ferric iron should be considered an independent component; although the point is debatable, the rarity of the staurolite-epidote assemblage (and presence of five phases in the ACFM system) suggests that this may be a truly univariant reaction of the type garnet + chlorite + muscovite = biotite + staurolite + epidote. Balanced reaction using the model compositions yields 0.383 garnet+ 0.505 chlorite+ 0.9 muscovite + 0.012 02 = biotite + 0.344 staurolite + 0.0575 epidote + 1.718 H20 + 0.893 quartz. This reaction yields a large amount of H20. In the section, Chloritoid-Stauro‘ite Relations (p. 30), I suggested that samples 355—1 and 356—1 were prob- ably formed under similar external conditions; ref- erence to figure 15 suggests that they may have formed in field S. Both samples 356—1 and 590—1 could have formed along the univariant line (Cd, Pg), but 356—1 seems more likely to have formed near the invariant point (Cd) itself. RELATION BETWEEN EPIDOTE-BEARING AND HORNBLENDE-BEARING ASSEMBLAGES Samples of three rocks in the garnet and stauro- lite zones have been found to contain hornblende. Of these, sample 161—1 is from just above the lower limit of the garnet zone, 102—1 is from just above the lower limit of the staurolite zone, and 289-2 is from well into the staurolite zone. The first two rocks were probably originally impure tufi'aceous sediments (161—1, in the allochthonous Everett For- mation, and 102—1 in the autochthonous Walloom- sac Formation). Sample 289—2 is from a highly calcareous pelite at the base of the Walloomsac, presumably originally in part a residual soil rich in aluminum and containing calcium. Rather excep- tional compositions are needed to cause hornblende to appear in the pelitic rocks, and a high calcium content seems to be required. The compositions of phases in these three specimens are summarized in table 9. The mineral assemblage-s including hornblende (table 1) suggest that a single phase volume in the ACFM projection—hornblende-garnet-biotite-chlo— rite (plus muscovite, plagioclase, quartz, ilmenite, and (or) magnetite)—suffices to account for the assemblages. As discussed by Doolan and others (1978), the apices of this phase volume shift pre— sumably in response to different metamorphic grades; sample 289—2 moreover has a highly mag- nesian biotite and is without chlorite, so that its assemblage occupies the three-phase “plane”———actu- ally a wedge-shaped volume—that is opposite the phase volume including chlorite. In the section, Phase Relations Involving Epidote (p. 34), the assemblage epidote-chlorite-biotite- garnet was shown to form a phase volume in the PROBLEMATIC PHASE RELATIONS ACFM projection. The Fe/ Mg ratio of hornblende from sample 161—1 is sufficiently high that the com- position of this hornblende may be within this vol- ume.8 The appearance of hornblende in 161—1, then, could be related to the reaction written as fol- lows, Where the actual compositions of phases from this rock (tables 3, and 9) as well as a model composition of biotite were used: 0.65 garnet+ 0.23 chlorite + 0.28 biotite + 0.64 epidote + 0.1 albite + 0.38 quartz = 0.25 muscovite + hornblende + 0.26 H2O + 0.11 02. The first appearance of hornblende apparently nearly corresponds to the petrographic garnet-zone marker. As the metamorphic grade increases, the compo- sition of the hornblende changes. Sample 102—1 was collected nearly 2 km east of sample 161—1 and is of higher grade; the compositions of all the phases of sample 102—1 are significantly different from those of sample 161—1 (table 9). On the basis of the microprobe data for these phases, the reaction involving the same phases must be written 0.99 epidote + 0.34 biotite + 0.60 chlorite + 0.4 albite + 1.08 quartz = 0.27 garnet+ hornblende + 0.31 muscovite + 1.91 H20 + 0.15 02. The chemographic relations now are qualitatively different, and the appearance of hornblende in this rock is of the tieline switch type. In between the two sample localities, the hornblende composition relative to those of the other participating phases must have changed so that hornblende became mo- mentarily coplanar with the phases epidote, biotite, and chlorite (and albite), but the exact point of that degeneracy is as yet undefined. It seems reasonable to suppose that the horn- blende compositional volume is a continuous one extending from within the epidote-chlorite-biotite- garnet (plus albite) volume to that corresponding to sample 102—1. One puzzle is that a rock (sample 590—1) about 5 km south-southeast of sample 102—1 but considerably upgrade from it and roughly iso- gradic with the rock of sample 289—1 contained this four-phase assemblage in which all phases are euhedral and groundmass phases are apparently in equilibrium. Because formation of hornblende from this four-phase assemblage involves consid- sSample 161—1 does not actually include biotite; what at first appeared to be biotite was proved by microprobe analysis to he oxychlorite (Chatter- jee, 1966). However, we can assume a wide range of biotite compositions and still define, together with the other minerals, a phase volume that would contain the composition of the hornblende of sample 161—1. 37 erable devolatilization, it seems improbable that the four-phase assemblage could be stable in rocks of higher grade than those containing hornblende, even if the boundary of the hornblende phase vol- ume in the ACFM projection can be assumed to have receded entirely outside of that phase volume. One possibility is that the value of “w in sample 590—1 may have been considerably higher than in 102—1 (the latter might have had appreciable CO2 dilution), so the reaction in 590—1 was retarded. This, however, is speculation. Another reaction leading to the formation of hornblende that is conceivable, though not observed in this area, involves the five phases epidote, gar- net, ,chlorite, hornblende, and chloritoid. Clearly this reaction would have to take place at higher grade than that corresponding to sample 161~1 because that assemblage represents the first appear- ance of hornblende and because the reaction involv- ing chloritoid must be of the tieline switch type. Using the mineral compositions for 102—1, the bal— anced reaction is: 0.838 epidote + 0.4 albite + 0.176 garnet + 0.741 chlorite + 0.482 quartz = hornblende + 0.652 chloritoid + 1.731 H20 + 0.11 02. After the appearance of staurolite and disappear- ance of chloritoid, another appropriate reaction would be approximately 0.918 epidote + 0.745 chlorite + 0.4 albite +0.716 quartz = 0.113 staurolite + 0.051 garnet + hornblende + 2.382 H20 + 0.13 02. Neither the hornblende-staurolite nor the horn- blende-chloritoid assemblage has been observed in the area: both, however, have been found else- where (Frey. 1974. p. 495 and quoting J. S. Fox, p. 498; Fed’kin, 1975, p. 75). CUMMINGTONITE-BEARING ASSEMBLAGES Several samples from two Widely separated locali- ties have yielded the assemblage quartz-almandine garnet-cummingtonite. One locality (487-2) is south of Wachocastinook Brook about 1 km southwest of Lions Head. The other locality (170—1) is on the south slope of Bear Mountain. Both are in the Bash- bish Falls Quadrangle, and the rocks are in the allochthonous Everett Formation (tables 1 and 2). The rocks are dense, fine grained, and pinkish gray (the pink hue is due to almandine) and contain scattered plumose olive-green fibrous clusters of cummingtonite. The sample from Bear Mountain 38 METAMORPHIC MINERAL ASSEMBLAGES, contains epidote in addition to garnet, quartz, and cummingtonite (fig. 6A). Feldspar is absent from these rocks, and muscovite, biotite, and chlorite are exceedingly rare; chlorite is present mainly as al- teration rims around garnet. This assemblage, which is virtually alkali free, is the only one found in which the garnet rim is also virtually calcium free; the garnet is rich in manganese, especially in the core, where manganese occupies as much as 15 per- cent of the divalent cation positions. The coexisting cummingtonite is virtually aluminum and calcium free, is very low in manganese, and has a Fe/Mg ratio of about 5/2. Thus, the mineral chemistry indicates that the rock is rich in silica and contains a major amount of iron and alumina but very little else. The protolith could have been saprolitic material. CONDITIONS OF METAMORPHISM Although quantitative estimates of the tempera— ture and pressure values of metamorphism across the area are difi‘icult to make, helpful estimates of approximate values can be made, largely on the basis of existing experimental studies of phase-stability relations in pertinent systems. Holdaway (1972) and Liou (1973) studied the stability of epidote in oxygen-buifered systems. Liou used several buifer systems, HM (hematite-magne- tite), NB (nickel-bunsenite), and QFM (quartz- fayalite—magnetite), and determined the maximum stability limits of epidote of specific compositions (Fe/Al=1/g for HM, =1/3 for NB and QFM) that are similar to the compositions of epidote from the area. The upper thermal stability of Liou’s epidote, therefore, should correspond to the upper limit for the metamorphism of epidote—bearing rocks. The oxygen fugacity of the rocks is unknown. The low- est grade rocks contain hematite, and higher grade rocks (below the garnet zone) contain magnetite; thus, presumably somewhere in the less metamor- phosed part of the chloritoid zone, the HM buffer surface intersects the land surface at its present level of erosion. Higher grade rocks probably were metamorphosed under more reducing conditions. Be- cause lower oxygen fugacity reduces the stability field of epidote, everything else being equal, the curve of Liou based on the HM—buffered system can truly be considered the upper limit of metamor- phism (fig. 18; compare curve 3'with curves 4 and 5). In figure 18, curves 1 and 2 show the phase diagrams for the aluminum silicate polymorphs ac- TACONIC ALLOCHTHON, MASS., CONN., N.Y. cording to Richardson and others (1969) and to Holdaway (1971), respectively. Kyanite is found in the study area. Sillimanite is a major rock-form- ing mineral in sillimanite-muscovite-garnet-quartz (with or without staurolite) associations in the “Ca— naan Mountain Schist” a few kilometers southeast of the present study area (Agar, 1932, 1933; D. S. Harwood, oral commun., 1975). If the kyanite and sillimanite formed roughly simultaneously, their stability boundary passed between the Taconic Range and Canaan Mountain. The pressure of over- burden in the area must have been no less than that corresponding to the triple point of the aluminum silicate polymorphs, about 3.8 kbar according to Holdway (1971) and about 5.5 kbar according to Richardson and others (1969). Day (1973) and Chatterjee and Johannes (1974) studied the upper thermal stability limit of stoi- chiometric muscovite in the presence of quartz. Their curves do not differ significantly, and figure 18, curve 13, shows that of Chatterjee and Johannes. Because muscovite—quartz is a nearly ubiquitous association in the area, the limit imposed on the stability of the association—assuming that the rela- tively small departures from stoichiometry of most of the muscovites may be ignored—is useful and restricts the temperature to lower values than those indicated by Liou’s (1973) work on epidote stabil- ity in HM-bufi'ered systems. A few rocks, especially at lower metamorphic grades, contain small amounts of paragonite (table 1) This phase has not been chemically analyzed and is identified only by the X-ray pattern. The basal spacings indicate that the paragonite is ap- proximately potassium free (Zen and Albee, 1964; Eugster and others, 1972) ; thus, the upper thermal stability of the paragonite—quartz assemblage stud- ied by Chatterjee (1972; this report, fig. 18, curve 12) is an approximate limit of the temperature at which paragonite in the study area formed. For a given total pressure and H20 activity, the highest temperature at which the paragonite—quartz assem- blage is stable is lower by about 100° C than the corresponding temperature for the muscovite—quartz assemblage. For a Pmm=PH20 of 4 kbar, the thermal limit is about 570° C for the paragonite-quartz assemblage. Many rocks at higher metamorphic grades con- tain staurolite and quartz. Staurolite is a phase of variable composition, and its stability may be affected by the oxygen fugacity. Therefore, Rich— ardson’s (1968) experimental results for staurolite of the specific composition HiFe4AlmSi7‘5048 in a 16 12 CONDITIONS OF METAMORPHISM 39 3-9 9° ." P’s-“P .“NT‘ 1 l | EXPLANATION Al-silicate polymorphs. Richardson and others, 1969. AI-silicate polymorphs according to Holdaway, 1971. Upper stability of epidote, HM buffer; epidote has pistacite (Ps)=0.33. Liou, 1973. Upper stability of epidote, NB buffer, Ps=0.25. Liou, 1973. Upper stability of epidote, QFM buffer, Ps=0.25. Liou, 1973. Staurolite+quartz=almendine+sillimanita+W, QFM buf- fer. Richardson, 1968. Staurolite+quartz=Fa-cordierite+sillimanite+W, GFM buffer. Richardson, 1968. Chloritoid+sillimanite+quartz=staurolita+W, (1PM buffer. Richardson, 1968. Chloritoid+oxygen=staurolite+magnetite+quartz+W, HM buffer. Gan uly and Newton, 1968 10. Chloritoid=Fe-cor i1=1rite+hercyniteSS ,NB buffer. Grieve and Fawcett, 1974. ' 108. Chloritoid=AI-Fe anthophyllite+staur’olite+hercynite, NB buffer. Grieve and Fawcett, 1974. 10b. Al-Fe anthophyllite+hercynite=almandine+staurolite+W, 6 NB buffer. Grieve and Fawcett, 1974. 11, 11a, 11b. Same as the corresponding reactions 10, 10a, 10b, OFM buffer. Grieve and Fawcett, 1974. 12. Paragonite+quartz=andalusite+albite+ W. Chatteriee, 9 1972. 128. Paragonite+quartz=kyanite+albite+W. Chatterjee, 15972.1 13. Muscovite+quartz=andalusite+sanidine+W. "Chatteriee 15 and Johannes, 1974. 13a. Muscovite+quartz=sillimanite+sanidine‘+‘ W. ~ Chatt‘erjee 108 and Johannes, 1974. 14. 0uartz+muscovite+Mg-chlorite=phlogopite+cordierite+ W. Seifert, 1970. 15. Mg-chlorite+muscovite=phlogopite+kyanite+quartz+w. Bird and Fawcett, 1973. 113 16. Daghnite=fayalite+Fe-cordierite+hercynite+W, MW uffer. Turnock, 1960. 12a 17. Daphnite=mullite+hematite+magnetite+quartz+W, HM buffer. Turnock, 1960. 18. Clinochlore=forsterite+cordierite+spinel+W. Chernosky, 1974. 19. Clinochlore+quartz=talc+cordierite+W. Chernoskyh1978. PRESSURE, IN KILOBARS on I /’< 1 // / 2 // / 19 12 14 s o 1 1 1 ‘8 1 1 \1 2 400 500 600 700 800 TEMPERATURE, IN DEGREES CELSIUS FIGURE 18.——Experimental pressure~temperature curves pertinent to the mineral assemblages of the study area. Right-i hand side of reactions 6 to 9 are high-temperature assemblages. W, H20. Buffer systems: HM, hematite-magnetite; NB, nickel-bunsenite; QFM, quartz-fayalite-magnetite; MW, magnetite-wiistite. QFM—bufl’ered system can be applied only with con- 1 curves 6 and 7) obtained by extrapolating Richard- siderable skepticism, especially because we do not | son’s (1968) results is consistent with those based yet have any thermochemical data to calculate the 3 on Liou’s (1973) epidote study or Chatterjee’s and effects of composition changes such as magnesium l Johannes’ (1974) muscovite—quartz study; the limit substitution for iron. Nevertheless, the upper limit I indicates that temperatures were less than about (staurolite + quartz = Fe-cordierite + sillimanite 650°—700° C for pressures of about 4 kbar during +H20, or =almandine+sillimanite+H20, fig. 18, I this reaction. 40 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. Another limit supplied by Richardson (1968) is the lower thermal stability limit for staurolite ac- cording to the reaction chloritoid+sillimanite+ quartz=staurolite+H20 in a QFM-buffered system (fig. 18, curve 8). The limit gives the lowest tem- perature at which staurolite is stable; other reac- tions leading to staurolite must be at higher tem- peratures when other conditions are equal. However, for H20 fugacity less than full load pressure, the curve would be shifted to lower temperatures, so again the limit is only approximate. For 4 kbar, the temperature given by Richardson is about 530° C, and the curve has a very steep slope. The curve intersects the kyanite-andalusite curve at about 4.5 kbar, according to Richardson and others (1969), or the kyanite-sillimanite curve at about the same pressure, according to Holdaway (1971). Ganguly (1968) and Ganguly and Newton (1968) studied the oxidation of chloritoid to staurolite+ quartz+magnetite under HM- and NB-buffer con- ditions, and Ganguly (1968) studied the reaction chloritoid + quartz # garnet + staurolite + H20, under the NB-buffer condition. As in Richardson’s (1968) experiments, the reactions refer strictly to the fer- rous end members of the phases; the natural ma- terials all show significant amounts of magnesium (plus calcium and manganese for garnet), and the reactions are not univariant. These reactions do supply estimates of the upper stability of the chlo- ritoid—quartz pair, at least approximately. Unfor- tunately, the reactions were run at pressures in ex- cess of 10 kbar and application to the study area most likely requires extensive extrapolation. Figure 18, curve 9, shows the HM-buffered data; extrapo- lation of the curve suggests that temperatures not greater than about 550° C at 4 kbar for stability of the chloritoid—quartz pair is indicated; these are not inconsistent with the estimates based on the lower thermal stability of staurolite. Grieve and Fawcett (1974) reported on the upper thermal stability of ferrochloritoid, using both NE and QFM buffers. The reactions they investigated are: chloritoid=Fe-cordierite+hercynite (low pres- sures) ; chloritoid = Fe—anthophyllite + staurolite + hercynite ; and Fe-anthophyllite + hercynite = alman- dine+staurolite+H20. The system is free of quartz, and thus the stability limit of chloritoid is a maxi- mum value. Their reactions (fig. 18, curves 10, 10a, 10b, 11, 11a, and 11b) can be compared with the esti— mated curve of Ganguly (1969) for chloritoid + quartz = staurolite + almandine + H20 (not shown on fig. 18). Ganguly’s quartz-saturated reaction oc- curs at about 50° C lower (for beginning of chlo— ritoid breakdown) to 100° C lower (for completion of staurolite+almandine reaction; these two are de- scribed as a single step by Ganguly Without inter- mediate anthophyllite) than those of Fawcett and Grieve. This difference could be consistent with the effect of excess quartz in reducing the stability field of chloritoid. However, because data are lacking, nothing more can be stated. The uncertainty re- garding the compositions of the phases involved in the different studies, especially the composition of staurolite, makes comparison even more hazardous. Fawcett and Yoder (1966) studied the upper thermal stability of the magnesium chlorite clino- chlore Mg5A128i3010(0H)g; Chernosky (1974) added new and improved data (this report, fig. 18, curve 18). Turnock (1960) gave preliminary results on the upper thermal stability of two aluminum-rich iron chlorites, daphnite Fe4.8Al2.4Si2.8(O,OH)m and pseudothuringite Fe._2A13_6Si2.2(O,OH)18, using both the magnetite-hematite and the magnetite-wiistite buffer systems. Hsu (1968) studied the upper sta- bility of iron-chlorite Fe4,514,0A13-,Si2_5,2.0(O,OH)13 plus quartz in a system buffered by iron—quartz— fayalite; he also studied these phases plus magnetite using the iron-magnetite, iron-wiistite, QFM, and NB buffer systems. Although the stability limit of chlorites intermediate in iron-magnesium composi- tions and for different aluminum contents has not been determined, the available data (Fawcett and Yoder, 1966; Turnock, 1960; Hsu, 1968) do pro— vide rough estimates of the upper limits for these intermediate chlorite compositions in the presence of quartz; these estimates are particularly useful because the chlorite—quartz assemblage is common. Turnock’s (1960) experimental data are shown in figure 18, curves 16 and 17. Bird and Fawcett (1973) and Seifert (1970) studied the upper thermal stability of the muscovite- clinochlore assemblage in the presence of quartz; Seifert studied the reaction to 'cordierite+phlogo- pite, and Bird and Fawcett studied the reaction to phlogopite+kyanite. Chernosky (1978) studied the reaction of clinochlore+quartz to talc+cordierite. Although the product assemblages are not found in the area, the widespread and common presence of the reactant assemblages allows me to form very useful estimates of maximum temperatures, espe- cially as the presence of iron-bearing chlorite should lower the limit of stability of the reactant assem- blage. Their results, plotted in figure 18, curves 14, 15, and 19, suggest that the mineral assemblages in the study area formed at temperatures of less than REFERENCES CITED 41 640° C at E 6 kbar, 580° C at 4 kbar, or 520° C at 2 kbar. A condition of metamorphism where P320 < __ >< __ __ __ __ __ 18—1 _____________ X __ __ __ __ __ X __ X __ X __ __ X __ .._ 19—1 _____________ ? __ __ __ __ X _- X __ X X __ __ __ __ Stp? 25—1 _____________ __ __ __ __ __ __ X X __ __ X X -_ __ __ __ 36—1 _____________ -_ __ __ __ __ __ __ __ __ __ X X X __ __ __ Palygorskiite 39—1 _____________ 9 X .._ __ _- X X __ X __ X __ __ X __ _. 40—2 _____________ __ X __ __ __ X X .._ X ? X __ __ __ X __ 43—1 _____________ .._ X __ __ X __ X __ X __ X __ __ __ ? __ 45—1 _____________ __ X X __ _.. __ X __ X __ X __ __ __ _... .._ 55—1 _____________ __ __ -_ __ _- __ X X X __ X X __ __ __ __ 55—2 _____________ __ __ __ __ __ __ X X X __ X X X __ _.. .._ 58—1 _____________ __ X __ __ __ __ X _.. X __ X __ __ __ .._ 59—2 _____________ __ X __ __ X __ X __ X __ X __ __ __ _.. .._ 59—3 _____________ '7 X _.. __ __ __ X __ X __ X __ __ __ __ __ 62—1 _____________ .._. X __ __ X X __ X .._ X __ __ _- X __ 63—1 _____________ __ X __ __ X __ X __ X __ X __ __ X __ .._ 64—1 _____________ __ X X __ __ __ X _... X .._ X __ __ X __ __ 65—1—2 ___________ __ X __ __ X __ ? __ X __ X __ __ __ ‘7 X 65—1—3 ___________ _.. X X _.. X X ? -_ X __ X __ __ __ '7 X 67—1 _____________ '7 X X __ __ __ X .._ X __ X __ __ __ __ .._ 67—2 _____________ .._ X __ __ __ X __ __ X X X __ __ _.. X __ 68—1 _____________ __ X X __ X X __ __ X __ X __ __ __ X __ 73—1 _____________ __ X X __ X X X -._ X __ X .._ __ __ X __ 77—1 _____________ X X X __ X .._. X __ X __ X __ __ __ .._ __ 77—2 _____________ __ X X __ X X X __ X __ X .._ __ __ __ __ 77—3 _____________ __ X X __ __ X X __ X __ X __ __ __ __ __ (78—1) ____________ __ X X __ __ __ __ __ X __ X __ __ __ __ __ 78—2 _____________ __ X X __ __ __ X __ X __ X __ __ __ __ __ 80—1 _____________ X X __ __ X __ X __ X __ X __ __ _.. __ __ 81—1 _____________ __ X __ __ X __ X __ X X X __ __ __ __ .._ 81—2 _____________ X X __ _.. X __ X .._ X __ X __ __ 7 __ __ 84—1 _____________ .._ X X __ __ -_ X __ X __ X __ __ X __ .._ 85—1 _____________ __ x x __ __ __ >< __ >< __ >< __ __ __ __ __ 90—1 _____________ __ x x __ __ __ x __ >< __ >< __ __ >< __ __ 90—2 _____________ X X __ __ X __ X __ X __ X __ __ _.. __ _.. 91—1 _____________ __ X X __ __ .._. X .._ X __ X __ _- X __ X 92—1 _____________ __ X X __ X _.. X __ X .._ X __ .._ __ __ __ 92—2 _____________ ‘7 X X __ X __ X __ X -_ X __ __ __ .._ X 93—1 _____________ .. X X __ __ __ X __ X _.. X __ __ X __ X 99—1 _____________ __ X X _.. X __ X __ X __ X __ __ __ __ __ 101—1 _____________ __ X X __ X __ X __ X __ X __ __ .._ X __ 101—2 _____________ __ X X __ X __ X __ X __ X _.. __ __ _.. X 102—1 ______________ X X __ __ X _.. X __ X __ X __ __ X X __ Hb 102—2 ___________ X X .._ __ X X X __ X __ X __ __ __ __ __. 103—1 _____________ X X X __ X .._ X __ X __ X __ __ X __ __ 103—2 _____________ X X X __ X __ X __ X __ X _.. __ X __ __ 107—1 _____________ __ X X __ X __ X __ X __ X __ __ X __ __ 107—2 _____________ __ X X __ X .._ X __ X _.. X __ _- __ __ __ 112—1 _____________ __ X X .._ X __ X __ X __ X _.. __ _- __ __ 120—1 _____________ __ X X __ __ __ X _.. X __ X .._. __ __ __ __ 50 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 1.—Mineral assemblages in samples of rocks from within and around the Tacom'c allochthon, southwestern Massa- chusetts and adjacent parts of Connecticut and New York—Continued Sample BI: (128—1) ____________ 128—2 _____________ x 129—1 ______________ _ Cd SI: Ga. Ep Pg Ksp Mu Pa Cc DoI Ilm M1: Tour Other X __ __ __ __ __ __ __ ? >< Zs I I I I I I I XXX I I X I I I I X I I I I 1 40—2 _____________ _ _ I I I I I XXX I I 161—1 _____________ __ __ __ X X X __ __ __ __ X X __ Hb ______________ X _.- _.. __ X __ __ _.. __ X __ _.- 163—1 _____________ __ X __ __ __ X __ __ __ __ X __ .__ 166-1 _____________ __ X __ __ __ X __ X __ __ _.. X __ 167—1 _____________ __ X __ __ __ X __ X __ __ __ __ _- (167—2) ____________ __ X __ __ _.. __ __ X __ __ __ __ __ 167—3 _____________ __ X __ X __ X _.. __ __ __ _.. __ __ -_ -_ __ Cum __ __ __ ? __ Cum,Ap I-‘ O} N) L Q xx xxxxx xxxxx xx:xx xxxxx g __ __ X __ X Stp? X I-‘I-l mm ‘I°‘I° NH I I I I I I I I I I I I I I I I I I I I I I I I I I I I II I I I I I I II I I I I I I I I I I I I XXXI I I I x x:xxx I I I «xxxx xxxm I I X __ __ _.. __ X Sliderite x IxxM I xxxx I | Stp? xx xxxxx xIx I I I I l I N) 00 N) L I I I I I I I l l I x x::xx xxxxx xxxxx xxxxx xxxxx x M q NI & I I I I I I I I I i X XX I I I I I I I XX XXXXX XXXXX XXXXX XXXXX XXXXX XXXXX XX} XX XXXXX XXXXX XXXXX I I [\3 4. 0‘0 03 E | I I l l I I I I I X I I I I I I __ __ __ X __ Hb m to co :0 f ? H N) ' l I l I I I I I I I l I l I l I I I I I I l I l I X X :xx m I I I I I x I X X I I I I X __ _.. -_ __ Z3 M {D ? H I I | I I I I I | I | I I X e X I I I I I I I I I xxxxx xxxxx xxxxx xxxxx xxxxx xxxxx xxxxx xxxxx xxxxx xxxxx xxxxx xxxxx xxxxx 0 I l I IIXXI | I I I NI 00 to [lg I I I I I I I I I x l I I l ><><: XI I wxxxx xxxxx xxgxx xIxIx xxx:x I I I I I I XXXXX XXX: X X l I CO CO p—A & I l I I l I I I I I I I x xxx: xxx x xxx l l TABLES 51 TABLE 1.—MI'ne’ral assemblages in samples of rocks from within and around the Taconic allochthon, southwestern Massa- chusetts and adjacent parts of Connecticut and New York—Continued Sample B1: 0 3‘ Cd St X n C) In PI ’5 Pg Ksp Mu Pa. Cc Dol Ilm Mt Tour Other M Ix xxxx I l «I I I I xxxx I l I I I xxm l I X I I I I I I __ __ __ __ >< Ap 03 a; 01 ’l‘ I I I I I I I I I I I I I : XXXXX XXXXX XXXXX X I I XXXXX XXX I I x l I I I I I I I wwxx xxxxx XQXI I I I I I I I I X I I I I __ -_ -_ __ __ Svtp,Zs | I I x xxxxx I I Sp‘h (375—1) ____________ __ I I I I I I I I I l I I I I ::::x xxxm I xxx xxxxx I l X I I I XX XXXXX XXXXX XX-7XX XXXXX XXXXX XXXXX D X I I I I I I I I CD 0': {D ’L I I I I I I I I I I I I I I XX XXXXX XXXXX XXXXI X CID oo ? H I I I I I I I I I | I I I I l I I I I I I I I I I I I X l I I | I I I I I I I I I XIIXXIIXXI I I I I I I I I X I I X I I I I _- __ __ __ __ Py 0:: 00 U1 I ._I I I I I I I I I I I X XXX I I I I I I 00 {9 If I-‘ I | I I I I I I I | I I I I I I I I | I ~xx:x xxxx l I I I I I I I I I I I .I; O U! h I I I I I I I I I X X XXXXX XXX I I I I I I I XXXXN? I I .h H O I H I I I | | I I I I I | I I | I I I I | I I I I I I I :xxxx xxxxx xxxxx xx:xx xxxxx xxxxx xxxxx xxxxx xxxxx xxxxx I I A I--I q I ._I I I I I l I I I I I I I I I I I I I I I I I I I I I I I I X I I X ’3 p—I I». I C I I I I I I I I l I XX I I I I I I I XX XXXXX XXX: X XXXXX I I I I I XX XXX I I I I Stp .p. a; a; 'ld I I I I I I I I I I I XXXXX XXXXX I I I I XX I I I I XXXXX XXXXX I I I I I I I I I X X I I pk C} I I-‘ I I I I I I I I I I I I ‘7 I I | I I I I I I I X __ __ __ Act I XXXXX XXXXX XXXQX XX"’XX XXXXX XXXXX X ‘7 I I I I I I I I II; IP- «1 o: co 6? L h I l I I I I I I I I I I I I I I I I I I l I I I I I x x Ixxx IIXM I I I I I :xxm I I xxxxg I I 52 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS, CONN., N.Y. TABLE 1,—Mineral assemblages in samples of rocks from within and around the Tacom'c allochthon, southwestern Massa- chusetts and adjacent parts of Connecticut and New York—Continued Sample Bt Ch Cd St Ga Ep Pg Ksp Mu Pa Cc Dol IIm Mt Tour Other iga X iga X iga X pl; 4 w L I I | I I I I I I I X XXX XXX | I A 00 ? H I I I I I I I I I I I I X I I I I I X __ __ __ __ S-ph? I xxxxx I l IP 00 q I N I I I I I I I I I X X X I I __ __ X __ Cum ,1; 00 I H I I I I I I I I I I I I X I I I I I I .p. t0 IQ N I I I I I I I I I I I : x xxx xx I I I xxx xxxx I I : xxx xix 01 Q | H I I I I I I l I I I I I | I I I I I I X | I I I I I I I I xx X XXXXX XI XXX XXXXX X I I I I X 01 H 00 ’L I I I I I I I I I I I: X X XXXXX X X I I 0101 on»; I? I-H-I I I I I I I I I I I I I I I I I I I I I I I I I I I XX I I I I I I I I I I I I 0" c3 {9 L I I I I I I I I I X X I I an (X) H J4 I I I I I I I I I I I X I I I I I I I XI IIIX IXXXI I I I I 01 00 m L I I I I I I I I I I I X I I I I I I I XXX: X XXXXX XXXXQ X I I U! 0‘ {D (X) o H k k I I I I I l I I I I I I l I I I I I l l I I I I I I x x x xxx x "E I xxx:x xxII xxx:x xx I I X ? __ __ __ Hem ‘I __ __ __ __ Dp,Trem 6:01 9:0 woo L& I I l I I I I I I l I I I l I I I I I I I l I l I | M M I I I I M W | I I I xx l I xxww xxx:x xxxxx xxxxx xxxxx xxxxw xxxxx | | C} o {D L I I I I I I I I X I I I I X X I I I I I I I I I I I I C3 0; O ’L I I I I I I I I I I I I X XXX I I I I XXX I XXXX X I I I I X I I Kyanibe O) II> {D L I I I I I I I I I X I I I I I I I I I I I I I I XXXX XXXXX I I X X X X .. _ _ X - _ X Fibrolite? _- X X X X X :xx:x I I xx I I Sph, Trem? __ __ __ __ Sph,Trem I I I I I I I I I I I I I I I I q o: 8 8 L L I | I | | I | I I I I l I I I I I I l I I I I I I I I I | I I I | I I I I I I I I I I I I l I I :xxxx I I I I :xxx I I I I __ __ -_ X Stp? I XXXXX XX: XX XXXXX XXXXX XXXXX XXXXX XXXXX XXXXX XXXXX XXXXX XXXXX D X X I I I I I I 11‘ H 01 £ I I I I I I I I I I I I I I I I I I I I I I I I I XXXX XXXXX I I I I I I X X I I I I I I I I I I X-=XX : I XXXXX XXXX I I I | I I : X X I I I I TABLES 53 TABLE 1.—Mine’ral assemblages in samples of rocks from within and around the Taoom'c allochthon, southwestern Massa- chusetts and adjacent parts of Connecticut and New York—Continued Sample BI; Ch Cd St Ga Ep Pg Ksp Mu Pa Cc Dol 11m Mt Tour Other __ __ __ Sph? I I XX-s'a I I I I I I __ __ __ Trem __ __ __ __ "x" Stp? >< Stp X __ __ _- __ X Stp? Sph "x" X I I XXX XXX: I I I I I I I I I Tourmaline vein I I I I I I I l I I l I l I I I I XXXXX X I I I I I I __ __ __ __ X By X __ __ __ __ Trem,Sph,Dp . . x x I XX xx-e: X . . 1087—1 _____________ __ I: I: I: I: __ I: I: I: I: I: I: I: (10'87—1—2) __________ __ __ -_ __ __ >< __ __ __ __ __ __ __ 1087—2 _____________ __ __ __ __ __ ? __ __ __ __ __ __ __ 'Q 1089—1 _____________ __ __ __ __ __ __ __ __ X X __ __ __ X X __ __ __ -_ __ __ X Stp? 1100—2 _____________ X __ __ __ X X __ X __ __ __ __ 1100—3 _____________ __ __ __ __ X X __ __ X __ __ __ X Stp? __ __ __ X Stp? I I XX-7XX XXXXX XXXXX XXXXX XXXXX XXXXN XXI I X I I 5—: )—l 0 <5 I H I I l I I I I I I I x xx: : x xxx: x ><><><><: I I I I I I (1115—1) ____________ __ __ __ __ __ __ __ X __ __ __ X __ __ _- (1115—2) ____________ __ __ __ __ _.. __ __ ? __ __ X __ __ __ (1115—3) ____________ __ __ __ __ __ __ __ X __ __ X __ __ _- 1118—1 _____________ __ __ __ __ __ X __ __ __ __ .. __ __ 1129—1 _____________ X _- __ __ __ X __ __ __ __ __ __ __ 1139—1 _____________ __ __ __ __ __ X __ __ __ __ I-J p—A 5b. 00 'ld| I I I I I I I I I I I I I XXXXX XX I I I I I I I I I I I I I I XXXXX XXXX Stp ? 32 __ >< __ __ Stp? 1180—1 _____________ __ __ __ __ __ __ __ __ __ __ __ __ 1189—1—1 ___________ __ >< __ __ __ __ >< __ __ >< __ >< 1189—1—2 ___________ __ __ __ __ __ __ __ __ __ >< __ __ __ __ __ __ x 12004 _____________ __ __ __ -_ __ __ __ __ __ __ __ >< Stp 1204—1 _____________ __ __ __ __ __ __ __ X __ _- __ >< Stp, Hem I XXX XXXXX XXXXX XXXXX XXXXX XX: XX XXXXX XXXXX XXXXX XXXXX XXXXX XX: XX 0 ._. ._. :9 N) iI_I : I I l I I I I I l I I : ><><><>< I x-ox: I I l I l I I l I I x><>< ><><><><>< ><><><><: I I I I l I I l I xxx xxx}: I I 54 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 2,—Localities from which samples were obtained for mineral-assemblage information [The availability of data from microprobe and wet-chemical analyses is indicated by footnote reference. Quadrangle: EGM, Egremont Quad- rangle, Mass.-N.Y.; CPE, Copake Quadrangle, N.Y.-Mass.; BBF, Bash- bish Falls Quadrangle, Mass.-Conn.-N.Y.; ASF, Ashley Falls Quadrangle, Mass.-Conn.; HDL, Hillsdale Quadrangle, N.Y.; STL, State Line Quad- rangle“ N.Y.-Mass.; MLT, Millerton Quadrangle, N.Y.-Conn: SHN, Sharon Quadrangle, Conn-N.Y.; GBR, Great Barrington Quadrangle, Mass. Formation: Ev, Everett Formation (allochthonous); W, Wal- loomsac Formation (autochthonous); W(Is), limestone in Walloomsac Formation; W(Eg), Egremont Phyllite, correlated with Walloomsac Formation (autochthonous); SB~a, SB—b. ‘”, SB—f Stockbridge Forma- tion. unit a, unit b, ”*2 unit f (autochthonous); Sb—brec, Stockbridge Formation, breccia (parautochthonous at base of the Taconic allochthon); Dal, Dalton Formation (autochthonous); W/Sb, on contact between Wal- loomsac and Stockbridge Formations. Zone: —Ga, below garnet zone; Ga, garnet zone; :Ga, at or near garnet zone lower boundary; +Ga, high—grade part of garnet zone; Ga/St, at or near garnet-staurolite zone boundary; St, staurolite zone: +St, high-grade part of staurolite zone. Metamorphic zone designations for samples from the Stockbridge Formation are extrapolated from zones in the pelitic rocks] Location _ Sample Quad- Long. Lat. Fgfla- Z0119 rangle 73° W+ N 3—1 ___- EGM 28’00" 42°09’37” EV —Ga 3—2 ___- EGM 27’57” 0-9’37” Ev —Ga 3—3 1 ___ EGM 27’53” 0‘9’36” Ev —Ga 3—5 _-__ EGM 27’45” 09’35” EV —Ga 3—6 ____ EGM 27’40‘" 09’33” Ev —Ga. 4—1 ____ EGM 28’39" 1022" W —Ga 14—1 1 ___ SHN 26’33” 41°59’48” W St 15—1 ___- BBF 27'03" 42°00'03" W St 16—1 ____ BBF 27’56” 00'44" Ev Ga/Sxt 16—2 ___- BBF 27’56” 00'44" Ev Gar/St 17—1 ____ BBF 28’01” 02’27” EV Ga 18—1 ____ BBF 27’59” 0‘4'17” W(Eg‘) ~—Ga. 19—1 ___- BBF 27’51” 04’52” W(Eg) —Ga 25—1 ___- EGM 25’36” 10"l6” SB—e Ga 36—1 ___- EGM 26’57" 0‘9’58” SB—c —Ga 39—1 ___- EGM 26’22” 08'38" EV —Ga 40—2 ____ EGM 25’54" 07’52” EV Ga 43—1 ___- BBF 26’01” 07’13” Ev Ga 45—1 1 ___ EGM 26’33” 0'8’45” EV —-Ga 55—1 ____ BBF 24’05" 06’43” SB—e Ga 55—2 ___- BBF 24’0’5” 06’41” SB—e Ga 58—1 ___- EGM 26’13” 08’36” EV +Ga 59—2 ____ EGM 26'08" 08’25‘" Ev +Ga. 59—3 ___- EGM 26’05” 08’24” Ev +Ga 62—1 ___- EGM 25’45” 07’43” EV +Ga 63—1 ___- EGM 25’53” 08’00” EV +Ga 64-1 ___- BBF 25’23" 06’33” EV Ga 65—1—2 __ BBF 25’37” 06'24" EV Ga 65—1—3 __ BBF 25’37” 06’24" Ev Ga 67—1 ____ BBF 25’59” 06’08” EV Ga 67—2 ____ BBF 25'53” 06’04” EV Ga 68—1 ___- BBF 26’12” 06’22” EV Ga 73—1 ___- BBF 26’14” 06’02” EV Ga 77—1 ____ BBF 24’52" 06’18” W Ga 77—2 ___- BBF 24'55” 06’18” EV Ga. 77—3 ___.-- BBF 25’07” 06’16” Ev Ga 78—1 ___- BBF 25'22” 06'19” Ev Ga 78—2 ____ BBF 25’22” 06’19” Ev Ga 80—1 ___- BBF 25'08” 0*5’42” Ev Ga 81—1 ___- BBF 2'5’03" 05’41” Ev Ga 81—2 ___- BBF 2’5’03” 0‘5'41‘” EV Ga 84—1 ____ BBF 25’22” 07’25” Ev Ga 85—1 _-_._ BBF 25’17” 07'17” EV Ga 90—1 ____ BBF 25’0‘8” 0‘5’19” Ev Ga 90—2 ____ BBF 25’0‘6" 05’21" Ev Ga 91—1 ___- BBF 25’27" 05'22” Ev Ga 92—1 ___- BBF 2‘5’42” 05'0 ” Ev Ga 92——2 ____ BBF 25’47” 04’58” Ev Ga 93—«1 ___- BBF 25’55" 0‘4’47” Ev Ga 99—1 ___- BBF 26’10" 03'45” Ev Ga/St TABLE 2.—Localities from which samples were obtained for mineral-assemblage information—Continued Location Sample Quad- Long. Lat. F33” Zone rangle 73° W+ N 101—1 ___- BBF 26’55” 42°03'21" EV Ga 101—2 ___- BBF 26’55” 03’21” EV Ga. 102—1 1 _-_ BBF 26’21" 03’0‘7" W Ga/St 102—2 ___- BBF 26'12” 03'16” W St 103—1 1 ___ BBF 25’58” 03'19” W St 103—2 1 ___... BBF 25’53” 0‘3’20” W St 107—1 ___._ BBF 26'15” 04'30” EV Ga 107—2 ____ BBF 26’03” 04'31" EV Ga 112—1 ___- BBF 26’05" 04'11” EV Ga 120-1 ___- BBF 27’20" 02’43” EV Ga. 128—1 ____ BBF 28’05” 07'09~ W(Eg) +Ga 128—2 ___.-- BBF 28’0‘5" 07’03" W(Eg‘) :Ga 129—1 ___- BBF 27’50” 07’05" W(Eg) Ga 130—1 ___- BBF 27’16” 03'50” EV Ga 131—1 ____ BBF 27’14” 03'56” EV Ga 140—1 ___- BBF 27’33” 0'5'0‘5" EV —Ga 140—2 1 2 _ BBF 27’43" 05’04" EV ——Ga 144—1 ___- BBF 28’10” 0542" W(Eg) —Ga 152—1 ___- BBF 28’04” 06'04" W (Eg) —Ga 161—1 1 ___ BBF 27'33” 03’23” EV Ga 162—1 ___- BBF 28’3‘6” 02’57" EV :Ga 163—1 ___- BBF 28’54” 03’08” EV —Ga 166—1 ____ BBF 28'59” 03’52” EV —Ga 167—1 ___- BBF 28'31” 03'45” EV —Ga 167~2 ___- BBF 28’31” 0‘3'45” EV —-Ga 167—3 ____ BBF 28’31" 03’45” EV —Ga 170—1 ___- BBF 27’22" 02’25" EV Ga 170—2 ___- BBF 27’22" 02'21” EV Ga 172—1 ___- BBF 28'01” 02'24" EV Ga 172—2 ___- BBF 28’01" 02’24” EV Ga 178—1 _-__ EGM 27’54" 08’5‘1” EV —Ga 182—1 ___- EGM 27'04” 08'02" EV —Ga 188—1 ___._ BBF 28’51” 0‘5'45” W(Eg) —Ga 188—2 ___- BBF 29’06” 05’42" W(Eg) —Ga 189—1 ___._ BBF 29’16” 05’42" EV —Ga 191—1 1 ___ BBF 29’04” 05’59" EV —Ga 195—1 ___- BBF 29’57” 06'58" EV —Ga 196—1 ___- BBF 29'50" 06'54” EV —Ga 208-1 ____ BBF 28'46" 04’13” EV —Ga 208—2 ____ BBF 28’05" 0‘4’17” EV —Ga 214-1 ___- BBF 26,06” 02'56” EV St 214—2 ___- BBF 26'06” 02’56” EV St 215—1 ____ BBF 26’38” 02'53" EV St/Ga 223—1 ___- BBF 29'53” 06’15” EV —Ga 223—2 ____ BBF 29’54” 06'07” EV —-Ga 224—1 ___-.. BBF 29’57” 05'57" EV —Ga 232—1 ___- BBF 26’35" 06’22" EV Ga 234—1 1 ___ BBF 25’0‘5” 04'54" W St 235—1 ___- BBF 25’30" 04,56" EV Ga 237—1 ___- CPE 30'01” 05’08” W —Ga 238-1 ___- CPE 30'18" 05’14" EV —Ga 238—2 -..__ CPE 30'11" 05’31" EV —Ga 242—1 ___- BBF 29'10” 02’23" EV ——Ga 24-3—1 ___- BBF 29’30” 02’20" EV —Ga 243—2 ___- BBF 2‘9'36” 02’30‘" EV —Ga 247—1 ___- BBF 24'56” 0‘5’41” EV Ga 247—2 ___- BBF 24'58” 05’44” EV Ga 250—1 ____ BBF 24’45” 0‘6’11” SB—f Ga 268—1 ____ EGM 24’15” 08'49” SB—c Ga 272—1 ____ BBF 26'16” 03'01" W St 272—2 ___- BBF 26’21” 03’0’4" W St 274—1 ____ BBF 25’59" 03’23" W St 2178—3 ____ BBF 24'25” 04’35" SB—b St TABLES 55 TABLE 2.—Localities from which samples were obtained for TABLE 2.—Localities from which samples were obtamed for mineral-assemblage information—Continued mineral-assemblage information—Cornnnued Location Location Forma- - ‘- Forma- Sam 1e - - Zone Sample uad- Lon . Lat. - Zone p fifii‘ie 7%01554 LIEIt. m“ gngle 73° W§+ N “on 289—2 1 ___ BBF 23’34” 42°03’05” W +St 421—3 ____ CPE 30’09” 42°00’19” Ev Ga 290—1 1 ___ BBF 23'32” 03’01” W +St 423—1 ____ MLT 30’03: 41:59:41: $7 ga 290_2 __-_ BBF 23,31” 03,01” W +St 429 1 ____ EGM 28 55 42 09 25 v 9. 291—1 ———— BBF 2‘3’13” 03’13” W +St 444—1 ___- SHN 29'59" 41°59'24" Ev Ga 297—1 _-__ BBF 25'15" 02’24" SB—b St 448—1 ____ SHN 29’27" 59'01” Ev Ga 298—1 ___- BBF 25’03” 02’03" SB—b St 456—1 _____ EGM 29'02" 42°10'13" Ev —Ga 298—2 ___- BBF 25’05” 02'22” SB—b St 4517-1 ____ EGM 2821” 1019” W —Ga 300__1 ____ BBF 2503» 0240” 83—1) St 463—1 1 ___ SHN 28’26” 41°58’21" W St 309—1 ___- EGM 22’32” 0’9’44” Dal Ga , u . » 331—1 1 BBF 26'24" 01'51" W St 233:1 33%“: 3223» 2331» E13 Ga§§t 331—3 ___- BBF 26:40:: 01:55:: EV St 466—2 _::: SHN 28'50" 59.04" EV Ga/St 333—1 _-__ BBF 27 22 01 37 EV Ga/St 468—1 ____ SHN 28'01” 58,55” EV St 336—1 ___- ggF 2653” 81,43" Ev Ga/St 473—1 ___- SHN 27’52” 58’01” W St 337—1 _-__ F 26’59” 1’11" Ev St , ,, , ,, 338—1 1 ___ BBF 27,20" 01:09» EV Ga/St 476—1 ___- SHN 27,03” 59,47" EV St 339_1 1 ___ BBF 26142” 00,28» EV St 478—]. ___... SHN 27 05 59 18 W St 340_2 « BBF 26:24” 00»320 W St 479—1 ___- SHN 27,29” 59,09" W St "" 486—1 _--_ SHN 25’43” 5914" W St 344—1 ___- BBF 28'28" 01’37” Ev Ga 487—1 -_-_ SHN 27’27” 5-9'55” Ev St 344—2 ___- BBF 28’37” 01’37” EV Ga 345—1 ____ BBF 2913" 0143” Ev Ga 487—2 1 ___ BBF 27’27" 42°00’02” Ev St 346—1 ___- BBF 29'44” 01'44” Ev Ga 488—1 ___- SHN 27’36” 41°58’17” W St 350—1 -___ BBF 2‘6’54" 00'25” Ev St 488—2 ___- SHN 27’36” 58’17” W St 495—1 _-__ SHN 28’49” 56’02‘" W Gav/ St 350—2 ———— BBF 26'54" 00'40” Ev St 496—1 ___- SHN 2905" 5.541" Ev Gar/St 352—1 ___- BBF 27’46" 00’0‘1” Ev S-t 352—3 ___- SHN 27’17” 41°59’57” Ev St 497—2 ___- SHN 29’14” 55’55” Ev Ga 355—1 1 2 __ BBF 2‘6’47” 42°00’25” Ev St 499—1 ____ SHN 28’32" 56’54” W St 356—1 1 2 __ BBF 26’11” 00'04" W St 504—1 ___- SHN 28’52” 56'15” W Ga/St 505—1 ___- SHN 28’57” 55’42" W(I:s) Grad/St 358-1 ___- SHN 25’54” 41°59’31" SB S‘t 506—1 1 ___ SHN 39’05” 55’13" Ev Gav/St 360—1 1 ___ BBF 29'20" 42°04'36” Ev —Ga 361—1 ___- CPE 30'04” 04’45” Ev —Ga 506—2 ____ SHN 29’14" 5‘5’09" Ev Ga 363—1 ___- BBF 29’46” 0‘5’16” Ev —Ga 509—1 1 ___ BBF 24’54” 42°05'38” W Ga 365—1 ___- BBF 29’40” 05’3‘9” Ev —Ga 513—1 ____ BBF 26’43" 02’50” Ev Gav/St 515—1 1 ___ BBF 26’13” 02’28” Ev St 366—1 ___- SHN 28'10” 41°59'40” W St 515—2 ___- BBF 26’08" 02’28" Ev St 366-2 ___- SHN 28’10” 5:9’37" Ev St 366—3 ___- SHN 28'10” 59’37” Ev St 517—2 ___- EGM 26’0‘9" 08'33” Ev Ga 369—1 1 ___ BBF 27'08” 42°00'30" Ev 51; 543—1 ___- BBF 24’16" 06’21” SB—c Ga 370—1 ____ BBF 2704” 0030" Ev St 563—1 ___- BBF 22’50” 03’28” W +St 569-4 ___- BBF 22'46" 05’22" W St 370—2 _-—— BBF 27’04" 0030” Ev St 581—1 ___- BBF 24’06" 0135'" W St 373—1 ___- HDL 30’18” 07’47” Ev —Ga 373—2 ____ HDL 30'27" 07'54" Ev —Ga 581—2 ____ BBF 24’08” 01’41” W St 375—1 ____ CPE 3044" 0639” Ev —Ga 588—1 -___ BBF 25’06” 0‘1’42” SB—b St 376—1—1 _ CPE 30"57" 06’22” Ev —~Ga 590—1 ~-—— BBF 24%” 00’44” W St 590—2 ____ BBF 24'26” 00’44” W St 376-1—2 _ CPE 30’57” 06’22” Ev —Ga 591—1 ___- BBF 24’35” 00’10” W St 376-1—3 __ CPE 30’57” 06’22” Ev —Ga 383—1 ___- CPE 31’40" 05'31” SB—brec —Ga 595—1 ___- BBF 23'56” 00'09” W +St 384—1 ___- CPE 31’00" 05’31” Ev —Ga 598—2 ___- ASF 21’22” 0‘4’07" SB—a +St 385—1 ___- CPE 30'35” 0‘4’36” Ev —Ga 603—1 ___- fill-31% 29’é0‘" 41°54‘11” 1%; Ga/SSt 609—1 ___- 23’ 3” 42°00’27” +1 1; 391—1 -__- BBF 29’51" 03’38” Ev —Ga _ I ~ r n v 233% “_— fig? 33,54" 04,03” SB —Ga 614 1 __-_ ASF 22 14 02 24 W +St — ___- ’36” 02’50” Ev —Ga _ ' ., ° , ' " 401—1 ___- BBF 2‘9'53" 02’28” Ev —Ga 626} "“ SH?! 2552,, 41059327,, W St , fl , " 630 1 ___- BB 25 52 42, 03 53 W Ga/St 401—2 ___- BBF 29 5'6 0‘2 27 EV «Ga 62%1 ____ £3; 25,47» 34,20» EV GaéS-t 6 —1 ___- B 24’47” 5’49" W 3. 401—3 ___- CPE 30’01” 02’26” Ev —Ga , ,, , ,, 405_2 ____ BBF 29:48» 01:52" EV :Ga 649—]. ___- ASF 21 37 02 18 W +St 405} "“ BBF 29,48” 01,52; E" iGa 650-1 ___- ASF 2119" 0245" W +St 406 1 ___- BBF 29 49 01 03 Ev Ga , » , » 407_1 CPE 30'07” 01,077: EV Ga 652—1 ___- ASF 21 11 0'1 42 W +St "" 655—1 1 ___ ASF 20’54” 02’21" W +St 407—3 -___ CPE 30'16” 01'07" Ev Ga. 658—1 ———— ASF 20%” 02’56” W +St 110—1 ___- CPE 3032" 0131" SB :Ga 659-1 ———- ASF 21'16” 01'15" SB +St 12—1 ___- BBF 28’27" 02’37” Ev :Ga 414.1 ____ CPE 3005» 04:23» Ev —Ga 677—1 -___ SHN 22’55” 41°59’42” W/SB +St 417.1 ____ CPE 31'59» 0429» SB —-Ga 677—2 ___- SHN 2‘2’55” 59’42" W/SB +St 690—1 ___- EGM 27’24” 42°12’15” Ev —Ga 421—1 ___- CPE 30’06" 00’19” Ev Ga 693—1 ___- EGM 27’29” 11’41” Ev —Ga 421—2 ___- CPE 30’02” 00’17” Ev Ga 701—1 ___- EGM 27’12” 12’34” SB—brec —Ga 56 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 2.—Localities from which samples were obtained for TABLE 2.—Localities from which samples were obtained for mineral—assemblage information—Continued mineral-assemblage information—Continued Location Location _ SW” 312”};73L°w+ N F‘t’i’o’“ SW” mi; NF°N+ ’33" F350? 2°“ 709—2 ____ EGM 29'04" 42°14'33" EV —Ga iggg-é—Z _ Egfi 32?: 42W111123”VV¥ —ga 715—1 ____ EGM 29’03” 14’29” v —-Ga — uw ’ ” ’ ” — a 715—2 ____ EGM 29’00” 14’26” SB—brec ——Ga , ,, , ,, 715_3 ____ EGM 28’59" 14,25» W —Ga 10‘87a3 ____ EGM 28,24” 11 53’ W —Ga 715_4 EGM 29,04» 14’31n SB—brec —Ga 1039—1 __.._. EGM 33 33 $2’00' SBfibrec ——ga “" 11 0—1 ____ PE ’ 1” 6’57” v — a 717—2 ____ HDL 30’09” 14’31" W(ls) —Ga 1100—2 ____ CPE 30’01" 06’57" Ev —Ga 749—1 ____ ASF 21’41” 06’50” Ev St 1100—3 ____ CPE 30’09” 06’59” Ev ——-Ga 749—2 ____ ASF 21’46” 06’43" EV St , ,, , ,, 750_1 ____ ASF 22,07" 06,54” EV St 110'2—1 ____ EGM 28 02” 12 46" EV —-Ga _ , n , » _ , 1106—1 ____ EGM 27’45 13’24 SBdbrec —Ga 753 2 ASF 21 29 05 47 SB a St 806—1 ____ EGM 25’19” 12’57” Ev Ga 11 7—1 ——__ ’ 6” ’ ” V — a 808—1 ____ EGM 24’20” 12'57" W(lsb) Ga 1114—1 ____ EGM 29'5‘6” 10’31” Ev —Ga 813—1 _-__ EGM 23'31” 11'17” SB— Ga , ,, , ,, 885~1 ____ EGM 26'17” 08’20” Ev Ga 1115—1 ____ HDL 30,16” 10 48” SB—b —Ga 893—1 EGM 25’08” 07'49" SB—c Ga 133:; ~——— EB}: 33 i5 £3: 3??) _ga ____ 1 __—— I 6” I II _ _ a 906—1 _-__ BBF 25’07” 07’19" SB—f Ga 1118—1 ____ EGM 29’41” 10:35” W —Ga 912—1 2.--- GBR 22’29” 08’17” SB-b Ga 1129—1 ____ EGM 29’43” 09 44'” W —Ga 918—1 __- EGM 22’43” 09’30" SB—a St , ,, , ,, 941_1 ____ GBR 22:00» 12,06” Dal Ga 1139—1 ___.. EGM 29,07” 09,05” EV —Ga 959—1 EGM 25’36” 14'03" Ev —Ga Elli-1 —-—— Egg 33 23 f)? 393 EV -ga ____ _1 ____ I II I II v _ a 967—1 ____ EGM 25'39” 14’31" SB-brec —Ga 1146—1 __—— EGM 28’38” 09’07” Ev —Ga 986—1 ____ EGM 2627” 1351" SB_d ——Ga 1147—1 ____ EGM 28’10" 12’36" Ev —Ga 994—1 ____ STL 29’24” 15’08” Ev —Ga 999—1 -___ EGM 29'26" 14’12” Ev ——Ga 1165—1 —_-_ HDL 31’28" 08’01” SB—d ——Ga 10‘00~1 ____ EGM 29’24" 14'10” Ev ——Ga Egg—i 1 ——— Egg 3830" (0);"33 EV —ga _ —__— I 7” I ‘ I} v __ a 1009—1 ____ STL 28’03” 22’24” EV —Ga 1173—1 ____ EGM 28’05" 11’06” W —Ga igfij __-_ Egg 33:3” fi’gé” EV —(G}a 1180—1 ____ EGM 27’32” 12’06" SB—brec —Ga ‘ ____ p )r r n V _ a 1019—1 ____ STL 29’03” 15’28” SB—brec —Ga 1189—1—1 _ EGM 28'21" 13’39" Ev —Ga 10234 ____ STL 29’02” 15’19" Ev vein —Ga 1189—1—2 _ EGM 28'21” 13’39" Ev ——Ga 1032—1 ____ EGM 26'54” 14'55" Ev —Ga 333:} ““ Egfi 33,22" 33,23» 136' :8: 1032—2 _.___ STL 26'48” 15’03" EV —Ga 1204_1 ____ EGM 28'19” 08’26” EV —Ga lggg—l ____ galfi 25’56" 15’03” Ev ——Ga "" 1 —1 _-__ 23’15” 41°59’55” W +St _ , » , n _ 1052—2‘ SHN 23'17" 59'58” W +8t 333.; 22:: 738% 33,33» 3713?], E3 £2 10,54_1 ____ SHN 22,33" 59,33» W/SB +S‘t 1216—1 ____ EGM 27’06” 14,06” EV —Ga 1073—1 ____ EGM 28’13” 42°13’00” Ev —Ga , . 1087—1 ____ EGM 28’28" 11/52" W —-Ga iWkih‘ifiifi‘Sailaéiblfiailable. TABLES 57 TABLE 3.—Microp’robe data on various minerals in samples of rocks from within and around the Taconic allochthon List of sections of table A. Muscovite B. Biotite C. Chlorite D. Chloritoid E. Staurolite F. Garnet G. Kyanite H. Homblende and Cummingtonite I. Epidote J. Plagioclase K. Ilmenite L. Magnetite [Both the chemical analyses and the number of atoms are based on the anhydrous formulas. Total iron is calculated as FeO. The excess oxygen equivalent to analyzed fluorine and chlorine has been subtracted from the sums and subsums of the analyses. The amount of water has been determined by converting the anhydrous mineral formula to the hydrous mineral formula and by considering the iron to be ferrous (see page 6 for details). Accuracy for major components is :1 percent; for minor components, it is :5 percent. Mineral assemblages are given in table 1; sample localities are given in table 2. nd, not determined. Abbreviations of mineral names: Bt, biotite; Ga, garnet; Pg, plagioclase; Ilm, ilmenite; Ch, chlorite; Cd, chloritoid; St, staurolite; Ep, epidote; Hb, hornblende; Mu, muscovite. Analyses of some spots mentioned in the notes are not included in table 3 because their sums are outside the louiz percent limit (see page 6 for discusson)] A. MUSCOVITE Sample ________________ 14—1 102—1 103—1 Spot ___________________ 15 19 042008 042009 051008 051009 051010 052002 Weight percent SiO; ______________ 42.57 44.57 44.89 44.93 45.30 45.46 46.42 45.19 TiOZ _____________ .38 .25 .20 .14 .25 .17 .31 .27 A1203 _____________ 37.05 35.12 35.33 35.68 35.30 34.51 34.89 35.07 FeO ______________ .84 1.14 1.46 1.33 1.67 2.23 2.23 2.00 MnO _____________ .01 .14 .03 .04 .02 .04 .03 .04 MgO _____________ .35 .30 .65 .58 .44 .72 .93 .49 03.0 _____________ .07 .00 .00 .0‘0 .00 .00 .00 .03 No.20 _____________ 1.36 1.58 1.19 1.13 1.3-0 1.38 .94 1.16 K20 ______________ 10.25 9.62 9.29 9.08 8.74 8.99 9.20 8.89 F ________________ .00 .20 nd nd nd nd nd nd Subsum ______ 92.88 92.75 93.04 92.91 93.02 93.50 94.95 93.14 H20 ______________ 4.37 4.39 4.41 4.42 4.43 4.42 4.50 4.42 Sum _______ 97.25 97.14 97.45 97.33 97.45 97.92 99.45 97.56 Number of atoms S11 _______________ 2.92 3.05 3.05 3.05 3.06 3.08 3.09 3.07 Ti _______________ .02 .01 .01 .01 .01 .01 .02 .01 A1 _______________ 2 99 2 83 2 83 2.85 2 83 2 76 2 74 2 81 Fe _______________ 05 06 08 .0‘8 09 13 12 11 Mn ______________ 0'0 01 00 .00 00 0‘0 00 00 Mg _______________ 04 03 07 .06 04 07 09 05 Ca _______________ 01 00 00 .00 00 0:0 00 00 Na ______________ 18 21 16 .15 17 18 12 15 K ________________ .90 .84 .81 .79 .75 .78 .78 .77 F ________________ .00 .04 mi nd nd nd nd nd Cation sum ___ 7.11 7.04 7.01 6.99 6.95 7.01 6.96 6.97 Notes Spots 042008 and 042009 Next to Next to are in same crystal Bt Ga 051011 052001: near Ilm 052003 58 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 3.—Microprobe data on various minerals in samples of rocks from within and around the Taconic alloehthon—Oominued A. MUSCOVITE—Continued Sample ________________ 1 03—1—Continued 103—2 161—1 Spot ___________________ 052004 052007 061011 061014 061017 062003 062010 1 2 Weight percent 8:102 ______________ 46.02 45.41 44.23 45.01 45.51 46.47 45.40 45.32 44.04 T102 _____________ .18 .24 .17 .27 .17 .20 .24 .46 .63 A1203 _____________ 35.05 34.71 35.49 34.56 34.48 32.66 35.00 35.29 37.20 FeO ______________ 2.08 2.62 1.94 2.03 1.91 1.89 1.96 2.57 2.14 MnO _____________ .02 05 06 .03 .05 .04 .04 .00 00 MgO _____________ 48 59 54 .91 .68 59 48 55 43 0210 _____________ 01 01 00 .03 .04 00 00 09 13 NaZO _____________ 67 1 06 1 29 1.02 1.03 93 1 31 71 68 2 ______________ 8 65 7 98 9 60 9.94 9.81 9 76 9 57 8 34 8 36 ________________ nd nd nd nd nd nd nd .00 .30 Subsum ______ 93.16 92.67 93.32 93.80 93.68 92.54 94.00 93.33 93.66 H20 ______________ 4.44 4.41 4.40 4.42 4.42 4.38 4.44 4.44 4.46 S-um _______ 97.60 97.08 97.72 98.22 98.10 96.92 98.44 97.77 98.12 Number of atoms Sa _______________ 3 11 3 09 3.02 3.06 3.09 3 18 3 07 3 06 2 97 T1 _______________ 01 01 .01 .01 .01 01 01 02 03 A1 _______________ 2 80 2 78 2.85 2.77 2.76 2 64 2 79 2 81 2 95 Fe _______________ 12 15 11 .12 .11 11 11 15 12 Mn ______________ 00 00 00 .00 .00 00 00 00 00 Mg _______________ 05 06 06 .09 .07 06 05 05 04 Ca. _______________ 00 0'0 00 .00 .00 00 00 01 01 Na ______________ 09 14 17 .13 .14 12 17 09 09 K ________________ 74 69 83 .86 .85 85 83 72 72 F ________________ nd nd nd nd nd nd nd .00 .06 Cation sum ___ 6.92 6.92 7.05 7.04 7.03 6.97 7.03 6.91 6.93 Notes Next to In Pg Next to B1: Bt 061018 062002 052008 and 062004 TABLES 59 TABLE 8.~—Microprobe data on various minerals in samples of rocks from within and around the Taconic allochthon—Cominued A. MUSCOVITE—Continued 191—1 234—1 290—1 331-1 1 2 4 1 2 3 091004 091006 092002 Weight percent 45.03 45.59 48.00 46.74 46.52 46.88 46.64 45.67 45.73 .20 .24 .38 .13 .25 .31 .21 .24 .30 35.80 34.78 37.73 37.66 36.12 36.46 36.55 36.93 36.53 3.45 3.07 .87 1.03 1.20 .91 1.27 1.07 1.66 .07 .00 .03 .01 .00 .00 .03 .04 .05 .32 .43 .48 .33 .61 .34 .49 .36 .52 .00 .01 .02 .02 .00 .02 .00 .00 .00 1.25 1.30 .87 1.66 1.36 1.71 1.88 2.02 1.90 8.69 8.62 8.77 8.54 8.46 8.62 7.42 7.82 7.92 .00 .00 .00 .00 .14 .00 nd nd nd 94.81 94.04 97.15 96.12 94.54 95.25 94.49 94.15 94.61 4.47 4.45 4.67 4.60 4.52 4.56 4.54 4.51 4.52 99.28 98.49 101.82 100.72 99.06 99.81 99.03 98.66 99.13 Number of atoms Si _______________ 3.02 3.07 3.08 3.05 3.08 3.09 3.08 3.04 3.04 Ti _______________ .01 .01 .02 .0‘1 .01 .02 .01 .01 .01 Al _______________ 2 83 2 77 2 85 2.89 2.82 2 83 2 84 2 89 2 86 Fe _______________ 19 17 07 .06 .07 05 07 06 09 Mn ______________ 00 00 00 .00 .00 00 00 00 00 Mg _______________ 03 04 05 .03 .06 03 05 04 05 Ca _______________ 00 00 00 .00 .00 00 00 00 00 Na ______________ 16 17 11 .21 .17 22 24 26 24 K ________________ 74 74 72 .71 .72 72 63 66 67 F ________________ .00 .00 nd .00 .03 .00 nd nd nd Cation sum ___ 6.98 6.97 6.90 6.96 6.93 6.96 6.92 6.96 6.96 Notes Next to Next to Next to Ch 11 Ch 111 091 005 091007 092001 60 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 3,—Microprobe data on various minerals in samples of rocks from within and around the Taconic allochthon—Conrtinued A. MUSCOVITE—Continued Sample ________________ 331—1—Continued 338—1 Spot ___________________ 092003 092009 093007 093012 101003 101005 101006 101007 102003 Weight percent 8102 ______________ 46.02 45.65 45.60 46.04 45.33 45.22 46.24 45.55 46.10 T102 _____________ .24 .19 .21 .26 .24 .17 .18 .23 .21 A1203 _____________ 37.05 37.31 36.55 36.43 35.12 36.59 35.29 36.46 35.98 F60 ______________ 1.08 1.09 1.26 1.23 2.19 1.58 2.151 1.57 2.10 MnO _____________ .11 .05 .05 .02 .04 .03 .05 .05 .04 MgO _____________ 41 .32 .43 .45 .64 .36 .56 .40 .49 03.0 _____________ 00 .00 .00 .00 .00 .00 .00 .00 .00 N320 _____________ 2.24 2.33 1.71 1.80 1.35 1.87 1.62 1.40 1.28 K20 ______________ 7.91 8.09 7.78 7.52 8.81 8.46 8.92 8.77 8.73 ________________ _ nd nd nd nd nd nd nd nd nd Subsum ______ 95.06 95.03 93.59 93.75 93.72 94.28 95.01 94.43 94.93 2 ______________ 4.55 4.54 4.49 4.50 4.44 4.49 4.50 4.49 4.56 Sum _______ 99.61 99.57 98.08 98.25 98.16 98.77 99.51 98.92 99.49 Number of atoms Sd _______________ 3.04 3.02 3.05 3.07 3.06 3.02 3.08 3.04 3.06 Ti _______________ .01 .01 .01 .01 .01 .01 .01 .01 .01 A1 _______________ 2.88 2.91 2.88 2.86 2.80 2.88 2.77 2.87 2.82 Fe _______________ .06 .06 .07 .017 .12 .09 .12 .09 .12 Mn ______________ .01 .00 .00 .00 .00 .00 .00 .00 .00 Mg _______________ .04 .03 .04 .04 .06 .04 .06 .04 .05 Ca _______________ .00 .00 .00 .00 .00 .00 .00 .00 .00 Na ______________ .29 .30 .22 .23 .18 .24 .21 .18 .17 K ________________ .67 .68 .66 .64 .76 .72 .76 .75 .74 F ________________ nd nd nd nd nd nd nd nd nd Cation sum __.. 7.00 7.01 6.93 6.92 6.99 7.00 7.01 6.98 6.97 Notes Next to Next to Near Next to Next to Next to Next to 1m St St Cd B1: crystal Bt 092001 093004, 093004 101002 101004 contain- 101008 and Ch 11 and Bt in: Mu 092004 093005 101001 101005 and Cd 093006 TABLES 61 TABLE 3.~Microprobe data on various minerals in samples of rocks from within and around the Tacom‘c allochthon——-Conrtinued A. MUSCOVITE—Continued 338—1—Continued 339—1 102004 102009 102013 102014 101101 111025 111029 112007 112008 Weight percent 46.33 46.41 45.76 46.37 46.16 46.47 46.70 46.52 46.18 .34 .24 .22 .20 .30 .18 .24 .19 .20 35.18 34.51 35.11 34.69 35.29 36.53 36.49 37.32 36.85 2.34 2.19 1.99 1.89 4.67 1.2.6 1.16 1.03 .91 .03 .02 .04 .0'3 .07 .03 .04 .02 .02 .71 .76 .62 .72 1.21 .5-6 .51 .55 .40 .00 .00 .00 .00 .03 .00 .00 .03 .00 1.17 1.15 1.40 1.24 1.03 1.98 2.01 1.74 1.97 8.56 8.92 9.21 9.02 8.08 8.22 8.60 8.50 8.34 nd nd nd nd nd nd nd nd nd 94.66 94.20 94.35 94.16 96.84 95.23 95.75 95.90 94.87 4.50 4.47 4.47 4.47 4.56 4.55 4.57 4.59 4.54 99.16 98.67 98.82 98.63 101.40 99.78 100.32 100.49 99.41 Number of atoms Si _______________ 3.09 3.11 3.07 3.11 3.04 3.06 3.07 3.04 3.05 Ti _______________ .02 .01 .01 .01 .02 .01 .011 .01 .01 Al _______________ 2 76 2 73 2 78 2.74 2.74 2 84 2 83 2.88 2 87 Fe _______________ 13 12 11 .11 .26 07 06 05 Mn ______________ 00 00 00 .00 .00 00 00 00 00 Mg _______________ 07 08 06 .07 .12 05 05 05 04 Ca _______________ 00 00 00 .00 .00 00 00 00 00 Na ______________ 15 15 18 .16 .13 25 26 22 25 K ________________ 73 76 79 77 .68 69 72 71 70 F ________________ nd nd nd nd nd nd nd nd nd Cation sum ___ 6.95 6.96 7.00 6.97 6.99 6.97 7.00 6.97 6.97 Notes Next to Between Spots 102013 and 102014 Next to Next to In Pg Ilm Cd are in same crystal Ilm St next to 102002 102005, 111024 111028 Ch Cd 112004 102008, and B1; 102010 62 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 3.—Microprobe data on various minerals in samples of rocks from within and around the Taconic allochthon—Continued A. MUSCOVITE—Continued Sample ________________ 355—1 356—1 Spot .................. 19 20 21 27 29 30 31 34 Weight percent 8102 ______________ 46.26 45.38 45.30 46.13 46.22 46.93 46.64 47.91 T102 ______________ .29 .31 .25 .29 .30 .20 .41 .15 A1203 _____________ 34.89 35.22 36.09 37.16 36.42 35.78 36.51 38.53 FeO ______________ 1.89 1.72 1.70 .89 1.24 .63 .71 .64 MnO _____________ .00 .03 .04 .03 .03 .00 .03 .04 MgO _____________ .38 .42 .39 .40 .55 .50 .38 .37 03.0 ______________ .00 .00 .00 .00 .00 .00 .00 .00 N820 _____________ 2.27 2.21 2.26 1.67 1.69 1.58 1.90 1.46 K20 ______________ 7.70 8.37 7.75 8.33 8.57 7.95 7.97 7.96 F ________________ .00 .37 .00 .00 .02 .00 .10 .61 Subsum _______ 93.68 93.72 94.10 94.90 95.82 93.57 94.57 97.16 H20 ______________ 4.47 4.45 4.47 4.55 4.55 4.51 4.54 4.68 Sum _______ 98.15 98.17 98.57 99.45 1010.37 98.08 99.11 101.84 Number of atoms Si ________________ 3.10 3.06 3.04 3.04 3.04 3.12 3.08 3.07 Ti ________________ .01 .02 .01 .01 .01 .01 .02 .01 A1 _______________ 2.76 2.80 2.85 2.89 2.83 2.80 2.84 2.91 Fe _______________ .11 .10 .10 .0'5 .07 .03 .04 .03 Mn _______________ .00 .00 .00 .00 .00 .00 .00 .00 Mg _______________ .04 .04 .04 .0‘4 .05 .05 .04 .03 Ca _______________ .00 .00 .00 .00 .00 .00 .00 .00 Na. _______________ .29 .29 .29 .21 .21 .20 .24 .18 K ________________ .66 .72 .66 .70 .72 .67 .67 .65 F ________________ .00 .08 .00 .00 .00 .00 .02 .12 Cation sum ___ 6.97 7.03 7.01 6.94 6.97 6.88 6.93 6.88 Notes Zn0=0.00 ZnO=0.00 Weight Zn0=0.00 Weight ZnO=0.00 Zn0=0.00 percept percent sum m- sum m- cluda eludes 0.32 0.80 ZnO: ca- ZnO; ca- tion tion _ sum in- sum m- cludes eludes 0.02 0.04 atom atom Zn Zn TABLES 63 TABLE 3.—Microprobe data on various minerals in samples of rocks from within and around the Tacom'c allochthon—Continued A. MUSCOVITE—Continued Sample ________________ 360—1 369—4 463—1 Spot ___________________ 121008 122005 122006 131015 132004 132007 141010 141011 142002 Weight percent SiOa ______________ 45.46 45.17 45.60 45.76 46.12 45.70 45.69 46.31 45.16 TiOa ______________ .2 .23 .36 .30 .38 .21 .20 .24 .17 A1203 _____________ 35.95 35.94 35.98 36.52 36.71 37.82 36.89 36.97 37.09 FeO ______________ 2.23 2.56 2.23 1.04 1.12 1.30 .99 1.02 1.15 M110 _____________ .07 .06 .05 .03 .02 .02 .04 .05 .02 MgO _____________ .27 .28 .28 .04 .36 .33 .36 .30 .30 09.0 ______________ .00 .00 .00 .00 .00 .00 .00 .00 .01 Na20 ____________ 1.57 1.33 1.37 1.87 1.95 1.92 2.17 2.05 2.16 K20 ______________ 9.03 9.05 9.96 8.44 8.27 8.24 7.85 7.98 8.37 ________________ nd nd nd nd nd nd nd nd nd Subsum ______ 94.78 94.62 95.83 94.00 94.93 95.54 94.19 94.92 94.43 2 ______________ 4.48 4.47 4.51 4.49 4.54 4.56 4.51 4.55 4.50 Sum _______ 99.26 99.09 100.34 98.49 99.47 100.10 98.70 99.47 98.93 Number of atoms Si ________________ 3.04 3.03 3.03 3.06 3.05 3.00 3.04 3.05 3.01 Ti _______________ .01 .01 .02 .02 .02 .01 .01 .01 .01 Al _______________ 2.84 2.84 2.82 2.87 2.86 2.93 2.89 2.87 2.91 Fe _______________ .12 .14 .12 .06 .06 .07 .06 .06 .06 Mn ______________ .00 .00 .00 .00 .00 .00 .00 .00 .00 Mg ______________ .03 .03 03 .00 .04 .03 .04 .03 .03 Ca _______________ .00 .00 00 .00 00 .00 .00 .00 .00 Na _______________ .20 .17 18 .24 25 .24 .28 .26 .28 K ________________ .77 .77 85 .72 70 .69 .67 .67 .71 F ________________ nd nd nd nd nd nd nd nd nd Cation sum --_ 7.01 6.99 7 05 6.97 6 98 6.97 6.99 6.95 7.01 Notes Next to Next to Next to In Bt Ch Ch Pg 142003 122004 132006 141012 64 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 3.—Microprobe data on various minerals in samples of rocks from within and around the Tacom'c allochthon—Continued A. MUSCOVITE—Continued Sample ________________ 466—1 506—1 Spot __________________ 151002 151006 152002 152004 153001 153002 172010 172012 172018 Weight percent 8102 ______________ 46.04 45.43 45.94 45.55 47.33 46.34 45.29 45.40 45.63 T102 _____________ .18 .13 .21 .20 .21 .22 .22 .23 .26 A1203 _____________ 35.59 36.84 36.36 37.14 35.91 36.56 36.25 36.38 316.48 FeO ______________ 1.66 1.48 1.28 1.25 1.33 1.11 1.42 1.10 1.02 MnO _____________ .03 .015 .03 .05 .03 .04 .04 .06 .04 MgO _____________ .95 .34 .41 .36 .41 .47 .51 .40 .43 03.0 _____________ .00 .00 .00 .00 .00 .00 .05 .011 .00 NaQO _____________ 1.04 1.96 1.82 1.73 1.75 1.85 1.66 1.28 1.85 K20 ______________ 8.77 8.39 8.73 8.82 8.56 8.57 8.26 8.50 8.42 F ________________ nd nd nd nd nd nd nd nd nd Subsum ______ 94.26 94.62 94.78 95.10 95.53 955.16 93.70 93.36 94.13 2 ______________ 4.49 4.51 4.51 4.53 4.56 4.54 4.47 4.46 4.49 Sum _______ 98.75 99.13 99.29 99.63 100.09 99.70 98.17 97.82 98.62 Number of atoms Si _______________ 3.07 3.02 3.05 3.02 3.11 3.06 3.04 3.05 3.05 Ti _______________ .01 .01 .01 .01 01 .01 .0‘1 .01 .01 Al _______________ 2.80 2.89 2 85 2.90 2 78 2.85 2.87 2.88 2.87 Fe _______________ .09 .08 07 .07 07 .06 .08 .06 .0'6 Mn ______________ .00 .00 00 .00 00 .00 .00 .00 .00 Mg ______________ .09 .03 04 .04 04 .05 .05 .04 .04 Ca _______________ .00 .00 00 .00 00 .00 .00 .00 .00 Na ______________ .13 .25 24 .22 22 .24 .22 .17 .24 K ________________ 75 .71 74 .75 72 .72 .71 .73 .72 F ________________ nd nd nd nd nd nd nd nd nd Cation sum _.__ 6.94 6.99 7 00 7.01 6 95 6.99 6.98 6.94 6.99 Notes Next to Next to Next to Next to Next to Next to Cd Bt Pg Ch Cd Ga. 151001 151005 152003 153003 172008 172013 and 172009 TABLES 65 TABLE 3.—Microprobe data on various minerals in samples of rooks from within and around the Taconic allochthon—C‘o‘nrtinued A. MUSCOVITE—Continued Sample ________________ 506—1—Con. 509-1 515—1 Spot .................. 174003 181002 181004 181008 182001 182004 182014 191003 191013 Weight percent S2i02 ______________ 45.52 45.54 46.07 46.42 46.58 47.05 46.11 45.28 46.58 Ti02 _____________ .26 .15 .17 .17 .19 .22 .17 .30 .25 A1203 _____________ 36.86 35.85 36.23 36.53 34.95 34.94 36.06 36.41 36.57 FeO ______________ 1.03 1.44 1.26 1.28 1.35 1.24 1.47 1.12 1.17 MnO _____________ .01 .04 .03 .01 .03 .03 .04 01 05 MgO _____________ 39 55 47 .50 83 56 49 47 49 0.210 _____________ .00 .00 .00‘ .00 .00 .01 .00 .03 .00 Nago _____________ 1.98 1.36 1.25 1.55 .98 .92 1.37 1.39 1.61 K20 ______________ 8.03 9.01 8.45 8.69 9.30 9.35 8.96 8.58 8.81 F ________________ nd nd nd nd nd nd nd nd nd Subsum ______ 94.08 93.94 93.93 95.15 94.21 94.32 94.67 93.59 95.53 H20 ______________ 4.50 4.47 4.49 4.54 4.49 4.50 4.51 4.47 4.56 Sum _______ 98.58 98.41 98.42 99.69 98.70 98.82 99.18 98.06 100.09 Number of atoms S1 _______________ 3 03 3 06 3 07 3.07 3.11 3 13 3 07 3.04 3.07 T1 _______________ 01 01 01 .01 .01 01 01 01 01 Al _______________ 2 89 2 84 2 85 2.84 2.75 2 75 2 83 2 88 2 84 Fe _______________ 06 08 07 .07 .08 07 08 06 Mn ______________ 00 00 00 .00 00 00 00 00 00 Mg ______________ 04 06 05 .05 08 06 05 0'5 05 Ca, _______________ 00 00 00 00 00 00 00 00 00 Na ______________ 26 18 16 .20 .13 12 18 18 21 K ________________ 68 77 72 .73 .79 79 76 73 74 F ________________ nd nd nd nd nd nd nd nd nd Cation sum ___ 6.97 7.00 6.93 6.97 6.95 6.93 6.98 6.95 6.98 Notes Next to Next to Next to Next to Next to Next to Outside Cd Bt Ch Cd Bt Ilm of Ga 181001 181003; 181007 182003 182013 191001 191004, and Bt with and Cd 191005, 181003 Mu 181001 191007, 181002 and and forms 3 181005 191012 Mu-Bt- Mu sand- 66 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 3.——Microprobe data on various minerals in samples of rocks from within and around the Tacom'c allochthon—Conrtinued A. MUSCOVITE——Continued Sample ................ 515—1—Continued 1052—2 1169—1 Spot __________________ 191016 191017 192003 1 26 211002 213005 213006 215003 Weight percent 8102 ______________ 46.44 45.61 46.04 46.17 47.25 48.57 47.19 49.45 48.31 T102 _____________ .28 .24 .30 .51 .55 .16 .16 .17 .17 A1203 _____________ 36.66 36.30 35.45 37.23 37.05 216.38 28.76 25.51 26.84 FeO ______________ 1.33 1.04 1.26 1.13 .89 4.66 4.34 4.37 4.94 M110 _____________ .02 .03 .04 .00 .06 .07 .06 .09 .09 MgO _____________ .47 .51 .54 .33 .33 2.67 2.57 2.83 2.38 08.0 _____________ .00 .02 .01 .02 .00 .00 .00 .0'0 .00 Na20 _____________ 1.69 1.64 1.36 1.46 1.32 .21 .26 .24 .29 K20 ______________ 8.66 8.97 8.64 8.89 8.98 9.39 10.44 10.55 10.32 F ________________ nd nd nd nd nd nd nd nd nd Subsum ______ 95.55 94.36 93.64 95.74 96.43 92.11 93.78 93.21 93.34 H20 ______________ 4.56 4.49 4.47 4.57 4.61 4.31 4.36 4.34 4.33 Sum _______ 100.11 98.85 98.11 100.31 101.04 96.42 98.14 97.55 97.67 Number of atoms 3.06 3.05 3.09 3.03 3.07 3.38 3.25 3.42 3.34 .01 .01 01 .03 .03 .01 01 .01 .01 2 84 2 86 2 80 2 88 2.84 2 16 2 33 2 08 2 19 07 06 07 06 .05 27 25 25 29 00 00 00 00 .00 00 00 01 01 05 05 05 03 .03 28 26 29 25 00 00 00 00 .00 00 00 00 00 22 21 18 19 .17 03 03 03 04 73 76 74 74 .74 83 92 93 91 nd nd nd nd nd nd nd nd nd Cation sum ___ 6.98 7.00 6.94 6.96 6.93 6.96 7.05 7.02 7.04 Notes Next to Next to Next to Next to Next to Next to Ch Ilm Ep Ep crystal Ilm 191014 192001 211001 213002, contain- 215001 and 1113 Mn and Ep 213004 213005 215002 TABLES 67 TABLE 3.—Micropvobe data on various minerals in samples of rocks from within and around the Taconic allochthon—Clominued B. BIOTITE Sample ________________ 14—1 102-1 103—1 Spot __________________ 3 14 18 20 043001 043002 051004 051005 051006 Weight percent S=i02 ______________ 33.39 33.73 33.35 35.59 34.07 34.80 33.89 33.77 33.51 T102 _____________ 1.87 1.96 1.95 2.08 1.81 1.87 1.61 1.56 1.65 A1203 _____________ 19.84 19.75 18.68 17.34 18.08 18.40 18.52 18.53 19.00 FeO ______________ 21.00 20.75 21.14 19.89 22.94 22.39 25.53 25.26 25.01 MnO _____________ .07 .00 .00 .00 .16 .14 .09 .09 .10 Mg'O _____________ 9.13 8.82 8.66 9.18 8.51 8.51 6.61 6.58 6.26 09.0 _____________ .01 .00 .27 .06 .09 .02 .07 .08 .06 Nan _____________ .30 .35 .38 .34 .10 .11 .28 .28 .30 K20 ______________ 7.77 9.31 8.08 9.21 7.88 8.27 8.42 8.30 8.13 F ________________ .57 .32 .30 .45 nd nd nd nd nd Subsum ______ 93.70 94.72 93.69 94.72 93.64 94.51 95.02 94.45 94.02 H20 ______________ 3.85 3.88 3.81 3.88 3.82 3.87 3.82 3.80 3.79 Sum _______ 97.55 98.60 97.50 98.60 97.46 98.38 98.84 98.25 97.81 Number of atoms S'i _______________ 2.60 2.61 2.62 2.75 2.60 2.70 2.66 2.67 2.65 Ti _______________ .11 .11 .11 .12 .11 .11 .10 .09 .10 Al _______________ 1.82 1.80 1.73 1.58 1.67 1.68 1.72 1.72 1.77 Fe _______________ 1.36 1.34 1.39 1.28 1.51 1.45 1.68 1.67 1.66 Mn ______________ 00 00 00 .00 .01 01 .01 .01 01 Mg ______________ 1 06 1 02 1 01 1.06 1 00 99 77 77 74 Ca _______________ 00 00 02 .00 .01 00 01 01 01 Na ______________ 04 05 06 .05 .01 02 04 04 05 K ________________ 77 92 81 .91 .79 82 84 84 82 F ________________ .14 .08 .07 .11 nd nd nd nd nd Cation sum ___ 7.77 7.85 7.81 7.80 7.71 7.78 7.83 7.82 7.81 Notes Weight ZnO=0.00 Weight Weight Spots 043001 and Next to In same percent percent percent 043002 are in same Cd crystal sum sum sum crystal 051001; as spot includes includes includes in same 051005 ZnO: ZnO: ZnO: crystal 0.23; 1.13; 0.96; as spot Cation Cation Cation 051006 sum sum sum includes includes includes 0.01 0.06 0.05 atom atom atom Zn Zn Zn 68 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 3.——Microprobe data on various minerals in samples of rocks from within and around the Tacom'c allochthon—Continued B. BIOTITE—Continued Sample ................ 103rl—Continued 103-2 Spot __________________ 051007 051011 052005 052006 052008 061001 061008 061010 1 Weight percent S‘i02 ______________ 33.37 33.94 33.51 33.29 33.76 33.25 33.20 33.61 36.55 Ti02 _____________ 1.76 1.64 1.64 1.85 1.52 1.72 1.70 1.55 1.50 A1203 _____________ 18.42 19.30 19.41 19.13 19.12 18.67 19.45 18.97 20.55 FeO ______________ 25.57 25.20 26.23 25.25 25.28 22.73 21.96 22.54 21.82 MnO _____________ .13 .13 .13 .12 .11 .11 .14 .11 .04 MgO _____________ 6.64 6.59 6.68 6.20 6.44 7.79 7.50 7.12 7.52 CaO _____________ .08 .01 .03 .01 .17 .02 .00 .08 .02 Na20 _____________ .30 .27 .27 .27 .29 .26 .21 .25 .21 K20 ______________ 8.03 8.57 8.70 8.31 7.98 8.95 9.00 8.71 9.06 F ________________ nd nd nd nd nd nd nd nd .28 Subsum ______ 94.30 95.65 96.60 94.43 94.67 913.50 93.16 92.94 97.31 H20 ______________ 3.79 3.85 3.86 3.80 3.82 3.78 3.79 3.78 4.01 Sum _______ 98.09 99.50 100.46 98.23 98.49 97.28 96.95 96.72 101.32 Number of atoms S‘i _______________ 2.64 2.64 2.60 2.63 2.65 2.63 2.63 2.67 2.73 Ti _______________ .10 .10 .10 .11 .09 .10 .10 .09 .08 A1 _______________ 1.72 1.77 1.78 1.78 1.77 1.74 1.81 1.78 1.81 Fe _______________ 1.69 1.64 1.70 1.67 1.66 1.51 1.45 1.50 1.36 Mn ______________ .01 .01 .01 .01 .01 .01 .01 .01 .00 Mg ______________ .78 .76 .77 .73 .75 .92 .88 .84 .84 Ca _______________ .01 .00 .00 .00 .01 .00 .00 .01 .00 Na ______________ .05 .04 .04 .04 .04 .04 .0‘3 .04 .03 K ________________ .81 .85 .86 .84 .80 .91 .91 .88 .86 F ________________ nd nd nd nd nd nd nd nd .07 Cation sum ___ 7.81 7.81 7.86 7.81 7.78 7.86 7.82 7.82 7.71 Notes Next to Next to Next to Next to Next to Next to Cd Mu Mu Cd 11 Cd 051001 051009 052007 061002 061007 061009 and 061002 TABLES 69 TABLE 3.—Microprobe data on various minerals in samples of rocks from within and around the Tacom‘c allochthon—Continued B. BIOTITE—Continued Sample ________________ 103—2—Continued 234—1 289—2 Spot __________________ 2 062008 062012 14 081001 081008 081013 Weight percent 8102 _____________ 36.06 33.94 33.61 33.70 35.19 35.10 36.36 Ti02 _____________ 1.43 1.76 1.79 2.07 1.99 2.03 1.86 A1203 _____________ 19.65 18.55 18.82 20.00 16.06 18.16 18.30 FeO ______________ 21.80 23.28 23.36 20.47 19.07 17.05 17.73 MnO _____________ .09 .12 .16 .01 .17 .20 .16 MgO _____________ 7.79 7.74 7.52 9.13 11.77 11.42 11.30 03.0 _____________ .02 .03 .01 .02 .09 .08 .07 Na¢0 _____________ .22 .18 .25 .21 .12 .22 .15 K20 ______________ 8.91 7.92 8.79 7.99 8.76 9.03 9.07 F ________________ .24 nd nd .32 nd nd nd Subsum ______ 96.01 93.52 94.31 93.65 93.22 93.29 95.00 H20 ______________ 3.95 3.81 3.82 3.87 3.85 3.90 . 3.97 Sum _______ 99.96 97.33 98.13 97.52 97.07 97.19 98.97 Number of atoms Si ________________ 2.74 2.67 2.64 2.61 2.74 2.70 2.75 Ti _______________ .08 .10 .11 .12 .12 .12 .11 A1 _______________ 1.76 1.72 1.74 1.83 1.47 1.65 1.63 Fe _______________ 1 38 1 53 1.54 1.32 1 10 1 21 1 12 Mn ______________ 00 01 .01 .00 01 01 01 Mg ______________ 88 91 .88 1.05 1 37 1 31 1 27 Ca _______________ 00 00 .00 .01 01 01 01 Na _______________ 03 03 .04 .03 02 03 02 K ________________ 86 80 .88 .79 87 89 87 F ________________ nd nd nd .21 nd nd nd Cation sum ___ 7.73 7.77 7.84 7.76 7.71 7.93 7.79 Notes Next to Next to Next to Next to Hb Cd Cd Pg 062007 062011 081007 081012 70 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 3.—-Microprobe data on various minerals in samples of rocks from within and around the Tacom'c allochthon—Continued B. BIOTITE—Continued Sample ________________ 290—1 331—1 338—1 Spot __________________ 4 5 6 093002 093003 101001 101004 101008 102010 Weight percent Si02 _____________ 34.71 34.18 33.91 34.90 35.06 34.02 33.58 33.99 33.20 T102 _____________ 2.11 2.21 2.18 2.05 2.22 1.82 1.67 1.70 1.75 A1203 _____________ 18.98 18.32 20.10 20.07 19.90 19.48 19.43 19.65 19.33 F60 ______________ 20.16 21.42 20.99 20.02 20.70 23.68 24.07 23.02 25.17 Mn-O _____________ .00 .00 .01 .09 .07 .09 .08 .10 .10 MgO _____________ 9.16 8.46 9.58 8.65 8.49 6.86 6.67 6.90 7.17 08.0 _____________ .00 .00 .00 .00 .00 .03 .05 .05 .04 N820 _____________ .36 .30 .27 .28 .28 .22 .25 .23 .18 K20 ______________ 8.96 9.00 7.74 8.24 8.28 8.52 8.63 8.84 8.06 ________________ .00 .00 .00 nd nd nd nd nd nd Subsum ______ 94.56 93.89 94.78 94.30 95.00 94.72 94.43 94.48 95.00 H20 ______________ 3.89 3.84 3.92 3.92 3.93 3.85 3.82 3.84 3.83 Sum _______ 98.45 97.73 98.70 98.22 98.93 98.57 98.25 98.32 98.83 Number of atoms Si ________________ 2.67 2.67 2.59 2.67 2.67 2.65 2.64 2.66 2.60 Ti _______________ .12 .13 .12 .12 .13 .11 .10 .10 .10 Al _______________ 1 72 1 69 1 81 1.81 1.79 1 79 1 80 1 81 1 79 Fe _______________ 1 30 1 40 1 34 1.28 1.32 1 55 1 58 1 50 1 65 Mn ______________ 00 00 00 .01 .00 01 00 01 01 Mg ______________ 1 05 99 1 09 .99 96 80 78 80 84 Ca _______________ 00 00 00 .00 00 00 00 00 00 Na _______________ 05 04 04 .04 04 03 04 03 03 K ________________ .88 .90 .75 .81 .81 .85 .87 .88 .81 F ________________ .00 .00 .00 nd nd nd nd nd nd Cation sum ___ 7.79 7.82 7.74 7.73 7.72 7.79 7.81 7.79 7.83 Notes Weight ZnO:0.00 Zn0:0.00 Spots 093002 and 093003 Next to Next to Next to Next to percent are in the same Cd Mu Mu Cd sum staurolite crystal 101002 101005 101007 102006, includes 093004 Mu ZnO: 102009; 0.12 in same crystal as spom 102011 and 102012 TABLES 71 TABLE 3.—Mz'crop'robe data on various minerals in samples of rocks from within and around the Tacom'c allochthon—Continued B. BIOTITE—Continued Sample ________________ 338—1—Continued 339-1 356—1 Spot ___________________ 102011 102012 101103 101105 111010 26 28 Weight percent 8202 _____________ 34.18 34.18 33.38 34.28 35.62 35.55 35.77 TiOa _____________ 1.77 1.70 1.55 1.5-3 1.75 1.39 1.45 A1203 _____________ 19.78 20.02 19.24 19.06 20.19 19.36 19.08 FeO _______________ 24.01 23.84 23.98 23.74 19.57 19.09 19.59 MnO _____________ .09 .09 .08 .12 .08 .00 .00 MgO _____________ 6 92 7.20 6.76 6.67 9.56 10.00 10.39 CaO _____________ 04 .03 .09 .04 .01 .00 .00 Na20 _____________ .28 .26 .17 .19 .29 .32 .34 K20 ______________ 8.77 8.86 8.02 8.39 8.55 8.00 7.86 ________________ nd nd nd nd nd .06 .34 Subsum ______ 95.84 96.18 93.27 94.02 95.62 94.08 94.53 2 ______________ 3.88 3.90 3.78 3.82 3.98 3.93 3.95 Sum _______ 99.72 100.08 97.05 97.84 99.60 98.01 98.48 Number of atoms 2.64 2.63 2.65 2.69 2.68 2.71 2.72 .10 .10 .09 .10 .10 .08 .08 1.80 1.82 1.80 1.76 1.79 1.74 1.71 1.55 1.53 1.59 1.56 1.23 1.22 1.24 .01 .01 .01 .01 .01 .00 .00 .80 .83 .80 .78 1.07 1.14 1.18 .00 .00 .01 .00 .00 .00 .00 .04 .04 .03 .0‘3 .04 .04 .05 .86 .87 .81 .84 .82 .78 .76 nd nd nd nd nd .01 .08 Cation sum ___ 7.80 7.83 7.79 7.77 7.74 7.73 7.74 Notes In same In same Next to Weight Zn0:0.00 crystal crystal Ga percept as spots as spots 111001— sum mt 102010 102010 111004 eludes and and and ZnO 102012 102011 Ch =O.36; 111011 cation} sum m- eludes 0.02 atom Zn 72 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 3.—Microprobe data on various minerals in samples of rocks from within and around the Tacom'c allochthon—Continued B. BIOTITE—Continued Sample ________________ 463—1 _ 466—1 487—2—4 Spot __________________ 141005 141006 141016 142003 142004 151005 1 2 Weight percent S4102 _____________ 33.37 34.52 34.10 34.31 34.34 34.57 34.99 34.81 Ti02 _____________ 1.72 1.78 1.59 1.61 1.60 1.21 .97 1.51 A1204 _____________ 18.47 18.57 19.20 19.24 19.92 19.91 15.88 16.78 FeO ______________ 21.81 23.44 22.61 21.88 21.53 22.29 29.77 30.18 MnO _____________ .15 .13 .12 .09 .10 .14 .08 .11 MgO _____________ 7.71 7.86 8.07 8.14 8.02 7.28 4.10 4.31 0510 _____________ .09 .04 .07 .04 .03 .00 .03 .09 Na20 _____________ .21 .21 .20 .25 .40 .39 .12 .07 K20 ______________ 8.17 8.13 7.86 8.30 8.67 6.92 8.79 8.67 F ________________ nd nd nd nd nd nd .05 .08 Subsum ______ 91.70 94.68 93.82 93.86 94.61 92.71 94.74 96.54 2 ______________ 3.75 3.86 3.84 3.85 3.88 3.83 3.72 3.79 Sum _______ 95.45 98.54 97.66 97.71 98.49 96.54 98.46 1070.33 Number of atoms Si ________________ 2.67 2.69 2.66 2.67 2.66 2.71 2.82 2.76 Ti _______________ .10 .10 .09 .09 .09 .07 .06 .09 Al _______________ 1.74 1.70 1.77 1.77 1.86 1.84 1.51 1.56 Fe _______________ 1.46 1.53 1.48 1.43 1.39 1.46 2.01 2.00 Mn ______________ .01 .01 .01 .01 .01 .01 .00 .01 Mg ______________ .92 .91 .94 .95 .92 .85 .49 .51 Ca _______________ .01 .00 .01 .00 .00 .00 .00 .01 Na _______________ .03 .03 .03 .04 .06 .0‘6 .02 .01 K ________________ .83 .81 .78 .83 .86 .69 .91 .87 F ________________ nd nd nd nd nd nd .01 .02 Cation sum _-_ 7.77 7.78 7.77 7.79 7.85 7.69 7.82 7.82 Notes Between Next to Bt 142003 and Bt 142004 Next to Cd Cd are in the same crystal Mu 141002 141007 and Bt 142003 is near 151006 and Ga Mu 142002 141015 TABLES 73 TABLE 3.—Mic’rop'robe data on various minerals in samples of rocks from within and around the Tacom'c allochthon—Continued B. BIOTITE—Continued Sample ________________ 609—1 Spot ___________________ 1 2 3 4 181003 182006 182007 182012 Weight percent S102 _____________ 36.74 36.46 37.15 37.21 34.60 34.71 35.96 34.95 T102 _____________ 1.54 1.46 1.35 1.57 1.67 2.11 1.75 1.61 A1203 _____________ 20.76 20.50 20.04 20.53 19.91 19.98 20.94 20.20 FeO ______________ 20.68 20.05 18.38 18.91 22.15 20.99 20.89 21.16 MnO _____________ .01 .04 .00 .00 .11 .11 .10 .11 MgO _____________ 7.99 8.25 8.57 8.67 8.29 8.19 8.22 8.31 0210 _____________ .02 .02 .03 .13 .04 .00 .01 .02 Na¢O _____________ .18 .32 .09 .04 .20 .23 .18 .26 K20 ______________ 7.74 8.61 8.7 8.62 8.26 8.69 8.51 8.83 ________________ .18 .42 .42 .44 nd nd nd nd Subsum ______ 95.69 95.78 94.43 95.75 95.23 95.01 96.56 95.45 2 ______________ 4.00 3.99 3.97 4.02 3.91 3.91 4.01 3.93 Sum _______ 99.69 99.77 98.40 99.77 99.14 98.92 100.57 99.38 Number of atoms Si ________________ 2.75 2.74 2.81 2.78 2.65 2.66 2.69 2.67 Ti _______________ .09 .08 .0‘8 .09 .10 .12 .10 .09 Al _______________ 1.83 1.82 1.78 1.81 1.80 1.80 1.85 1.82 Fe _______________ 1.29 1.26 1.16 1.18 1.42 1.35 1.31 1.35 Mn ______________ .00 .00 .00 .00 .01 .01 .01 .01 Mg ______________ .89 .92 .96 .96 .95 .94 .92 .95 Ca _______________ .00 .00 .00 .01 .00 .00 .00 .00 Na _______________ .02 .05 .01 .00 .03 .03 .03 .04 K ________________ .74 .83 .84 .82 .81 .85 .81 .86 F ________________ .04 .10 .10 .10 nd nd nd nd Cation sum ___ 7.61 7.70 7.64 7.65 7.77 7.76 7.72 7.79 Notes Next to Next to Next to Ddu Ch Ga 181002, 181007 182009, Mu 182010, 181004, and and Cd 182011 181001 74 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 3.—Mieroprobe data on various minerals in samples of rocks from within and around the Tacom'c allochthon—Con-tinued B. BIOTITE—Continued Sample ________________ 1052—2 Spot __________________ 27 28 29 Weight percent Si02 _____________ 35.40 33.97 34.13 Ti02 _____________ 2.17 2.41 2.15 A1203 _____________ 19.11 19.11 17.95 FeO ______________ 18.57 19.76 20.77 MnO _____________ .03 .01 .01 MgO _____________ 9.68 9.67 9 68 03.0 ______________ .00 .05 00 N 3.20 _____________ .21 .29 22 K20 ______________ 9.96 9.93 9 40 ________________ .44 .29 19 Subsum ______ 95.20 95.25 94.34 2 ______________ 3.94 3.90 3.85 Sum _______ 99.14 99.15 98.19 Number of atoms 2.70 2.61 2.66 .12 .14 .13 1 71 1.73 1.65 1 18 1.27 1.35 00 .00 00 1 10 1.11 1 12 00 .00 00 03 .04 03 97 .97 93 11 .07 05 Cation sum ___ 7.81 7.87 7.87 Notes Zn0=0.00 ZnO=0.00 Zn0=0.00 TABLES 75 TABLE 3.—Microprobe data on various minerals in samples of rocks from within and around the Taconic allochthon—Con-tinued C. CHLORITE Sample ................ 14—1 102—1 103—1 103—2 Spot __________________ 9 041005 041006 041007 042005 052011 2 3 Weight percent 24.05 24.51 25.05 24.31 24.82 22.15 22.30 23.17 .13 .13 .00 .10 .14 .00 .09 .05 23.80 21.67 21.64 21.93 22.88 22.80 23.87 22.75 26.71 26.97 26.63 26.11 26.17 31.63 29.42 28.94 .00 .14 .21 .15 .10 .12 .16 .12 MgO _____ ‘ ________ 12.47 14.01 14.59 14.48 14.10 9.44 11.33 11.90 CaO ______________ .02 .06 .00 .04 .04 .02 .00 .02 NaaO _____________ .07 .00 .00 .00 .00 .00 .00 .04 K20 ______________ .00 .00 .00 .00 .00 .00 .15 .15 F ________________ .00 nd nd nd nd nd .01 .03 Subsum ______ 87.27 87.49 88.12 87.12 88.25 86.16 87.32 87.14 H20 ______________ 11.26 11.24 11.36 11.24 11.43 10.74 11.02 11.03 Sum _______ 98.53 98.73 99.48 98.36 99.68 96.90 98.39 98.17 N umber of atoms Si _______________ 2.56 2.62 2.65 2.59 2.61 2.48 2.43 2.52 Ti _______________ .01 .01 .00 .01 .01 .00 .01 .00 Al _______________ 2.99 2.73 2.69 2.76 2.83 3.00 3.06 2.91 Fe _______________ 2.38 2.41 2.35 2.33 2.30 2.96 2.68 2.63 Mn ______________ .00 .01 .02 .01 .01 .01 .01 .01 Mg ______________ 1.98 2.23 2.30 2.30 2.21 1.57 1.84 [.93 Ca _______________ .00 .01 .00 .00 .00 .00 .00 .00 Na _______________ .01 .00 .00 .00 .00 .00 .00 .01 K ________________ .00 .00 .00 .00 .00 .00 .02 .02 F ________________ .00 nd nd nd nd nd .00 .01 Cation ‘sum ___ 9.93 10.02 10.01 10.00 9.97 10.02 10.05 10.03 Notes Weight Next to Next to percent crystal Hb sum in- contain- 042006 eludes ing Ch ZnO: 041005 76 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS, CONN., N.Y. TABLE 3,—M’i0'roprobe data on various minerals in samples of rocks from within and around the Tacom'c allochthon—Continued C. CHLORITE—Continued Sample ________________ 161—1 191—1 234—1 331—1 Spot __________________ 4 5 6 1 2 6 091005 091007 092004 Weight percent SiOz _____________ 24.46 26.60 24.42 22.56 22.77 24.00 23.71 22.99 24.93 Ti02 _____________ .07 .04 .00 .04 .11 .16 .07 .0‘0‘ .17 A1203 _____________ 27.05 24.67 27.15 23.77 22.76 21.79 23.47 23.71 24.68 FeO ______________ 30.05 29.33 29.46 30.55 30.93 29.00 28.1 27.75 27.80 MnO _____________ nd nd nd .66 .63 .02 .07 .09 .08 MgO _____________ 10.98 11.81 10.73 10.18 10.09 11.95 12.04 11.56 11.93 CaO ______________ .06 .06 .02 .05 .02 .02 .00 .00 .00 Na20 _____________ .02 .00 .08 .02 .01 .06 .00 .00 .00 K20 ______________ .00 .00 .00 .13 .13 .00 .02 .00 .12 F ________________ .19 .19 .00 .00 .04 .02 nd n-d nq_ Subsum ______ 92.72 92.54 91.86 87.96 87.46 87.00 87.49 86.10 89.71 H20 ______________ 11.86 11.92 11.79 11.02 10.92 11.05 11.19 11.01 11.56 Sum _______ 104.58 104.46 103.65 98.98 98.38 98.05 98.68 97.11 101.27 Number of atoms Si _______________ 2.47 2.67 2.49 2.46 2.50 2.60 2.54 2.51 2.59 Ti _______________ .01 .00 .00 .00 .01 .01 .01 .00 .01 A1 _______________ 3.22 2.92 3.26 3.05 2.95 2.79 2.97 3.05 3.02 Fe _______________ 2.54 2.47 2.51 2.78 2.84 2.63 2.52 2.53 2.41 Mn ______________ nd nd nd .06 .06 .00 .01 .01 .01 Mg ______________ 1.65 1.77 1.63 1.65 1.65 1.93 1.92 1.88 1.84 Ga _______________ .01 .01 .00 .00 .00 .00 .00 .00 .00 Na _______________ .00 .00 .0‘1 .00 .00 .01 .00' .00 .00 K ________________ .00 .00 .0'0 .02 .02 .00 .00 .00 .02 F ________________ .06 .06 .00 .00 .01 .01 nd nd nd Cation sum -__ 9.90 9.84 9.90 10.02 10.03 9.97 9.97 9.98 9.90 Notes Samples from 161—1 could be partially Zn0:0.00 Next to Next. to Next to oxidized to oxychlorite, and assign- Mu Mu Mu ment of H20 may be too high, lead- 091004 091006 092003 ing to high oxide sums and low 3.321100111m cation sums TABLES 77 TABLE 3,—Mic’roprobe data on various minerals in samples of rocks from within and around the Taconic allochthon—Gontinued C. CHLORITE—Continued Sample ________________ 331-1—Continued 339—1 360—1 Spot __________________ 093005 093011 111011 111012 111013 111019 112004 122004 122008 Weight percent 8102 _____________ 23.29 23.28 24.39 24.13 24.14 23.42 23.64 22.01 22.45 T102 _____________ .08 .00 .00 .00 .08 .00 .09 .03 .08 A1203 _____________ 23.89 23.62 23.89 24.08 23.32 23.50 23.74 23.09 22.87 FeO ______________ 27.82 27.72 26.45 26.15 26.21 27.16 27.93 34.14 34.60 MnO _____________ .07 .06 .03 .04 .05 .07 .02 .45 .48 MgO _____________ 12.14 11.86 13.43 14.06 13.34 12.65 12.36 7.24 7.36 CaO ______________ .02 .01 .01 .01 .05 .04 .04 .01 .03 Naeo _____________ .00 .00 .00 .00 .00 .00 .00 .03 .02 K20 ______________ .00 .00 .00 .00 .15 .00 .02 .02 .04 ________________ nd nd nd nd nd nd nd nd nd Subsum ______ 87.31 86.55 88.20 88.47 87.34 86.84 87.84 87.02 87.93 H20 ______________ 11.17 11.08 11.41 11.45 11.27 10.81 11.25 10.68 10.78 Sum _______ 98.48 97.63 99.61 99.92 98.61 97.65 99.09 97.70 98.71 Number of atoms Si _______________ 2.50 2.52 2.56 2.53 2.57 2.52 2.52 2.47 2.50 Ti _______________ .01 .00 .0'0 .00 .01 .00 .01 .00 .01 A1 _______________ 3.02 3.02 2.96 2.97 2.92 2.98 2.99 3.06 3.00 Fe _______________ 2.50 2.51 2.32 2.29 2.33 2.44 2.49 3.21 3.22 Mn ______________ .01 .01 .00 .00 .00 .01 .00 .04 .04 Mg ______________ 1.94 1.92 2.10 2.19 2.11 2.03 1.97 1.21 1.22 Ga _______________ .00 .00 .00 .0‘0 .01 .00 .00 .0‘0 .00 Na _______________ .00 .00 .00 .00 .00 .00 .00 .01 .01 K ________________ .00 .00 .00 .00 .02 .00 .00 .00 .01 F ________________ nd nd nd nd nd nd nd nd nd Cation sum ___ 9.98 9.98 9.94 9.98 9.97 9.98 9.98 10.00 10.01 Notes Next to Next to Next to Next to Next to In Pg Next to St 1‘. B1; Ga. Ga nexé to 11 093004, 093010 111010 111001— 111001— Pg 122005 Cd 111004 111004: 112003 093006, not in and Mu an Mu same 112007 093007 crystal as spot 111012 78 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 3.—Microprobe data on various minerals in samples of rocks from within and around the Tacom'c allochthon—Continued C. CHLORITE—Continued Sample ________________ 369—1 463—1 466—1 Spot .................. 1 131006 132003 132006 1 2 141017 141018 151007 Weight percent Si_02 _____________ 22.63 23.45 22.79 23.26 22.26 23.16 23.20 23.62 23.40 T102 _____________ .20 .10 .05 nd .21 .11 .17 nd nd A1203 _____________ 24.72 23.88 23.75 23.27 25.00 24.40 23.42 23.46 23.19 FeO ______________ 28.09 27.48 29.00 29.18 27.63 27.18 28.49 28.79 30.88 MnO _____________ .01 .07 .05 .07 .14 .11 .15 .13 .20 MgO _____________ 11.55 10.89 11.20 11.47 11.71 11.82 11.51 11.519 10.55 CaO ______________ .02 .04 .00 .00 .00 .05 .00 .01 .00 Na20 _____________ .00 .00 .00 nd .00 .00 .00‘ n:d nd K20 ______________ .14 .02 .02 nd .14 .17 .05 nd nd F ________________ nd nd nd nd nd nd nd nd nd Subsum ______ 87.36 85.93 86.86 87.25 87.09 87.00 86.99 87.60 88.22 H20 ______________ 11.14 11.02 11.02 11.07 11.12 11.15 11.07 11.16 11.10 Sum _______ 98.50 96.95 97.88 98.32 98.21 98.15 98.06 98.76 99.32 Number of atoms 2.43 2.55 2.48 2.52 2.40 2.49 2.51 2.54 2.53 .02 .01 .00 nd .02 .01 .01 nd nd 3.13 3.06 3.05 2.97 3.18 3.09 2.99 2.97 2.95 2.52 2.50 2.64 2.64 2.49 2.44 2.58 2.59 2.79 .00 .01 .00 .01 .01 .01 .01 .01 .02 1.85 1.77 1.82 1.85 1.88 1.89 1.86 1.86 1.70 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 nd .00 .00 .00 nd nd .02 .00 .00 nd .02 .02 .01 nd nd nd nd nd nd nd nd nd nd nd Cation sum ___ 9.97 9.90 9.99 9.99 10.00 9.95 9.97 9.97 9.99 Notes Next to Next to Next to Spots 141017 and 141018 Ga St Mu are in same crystal 131007 132002 132007 TABLES 79 TABLE 3.—-Microprobe data on various minerals in samples of rocks from within and around the TacorLic allochthon—Continued C. CHLORITE—Continued Sample ________________ 466—1—Continued 506 —1 609—1 Spot __________________ 152005 152006 153008 172004 172005 172017 181007 181009 182016 Weight percent Si02 ______________ 23.38 23.07 23.73 23.42 23.01 25.71 24.05 24.63 23.817 Ti02 _____________ .12 nd .16 .09 nd nd .07 .10 .15 A1203 _____________ 23.62 23.43 23.92 23.61 23.319 24.76 22.97 21.70 23.34 FeO ______________ 29.57 29.37 28.90 28.12 27.63 26.81 28.22 28.08 27.43 MnO _____________ .23 .22 .20 .08 .06 .03 .10 .09 .10 MgO ____________ — 10.53 10.85 10.24 11.84 11.95 10.50 12.55 12.61 12.24 08.0 _____________ .00 .00 .00 .01 .01 .02 .00 .01 .02 NaaO _____________ .00 nd .00 .00 nd nd .02 .00 .02 20 ______________ .30 nd .09 .05 nd nd .07 .05 .61 _______________ - nd nd nd nd nd nd nd nd nd Subsum ______ 87.75 86.94 87.24 87.22 86.05 87.83 88.05 87.27 87.78 2 ______________ 11.08 11.01 11.11 11.14 11.00 11.43 11.24 11.15 11.19 Sum _______ 98.83 97.95 98.35 98.36 97.05 99.26 99.29 98.42 98.97 Number of atoms Si ____________ -___ 2.53 2.51 2.56 2.52 2.51 2.70 2.56 2.65 2.55 Ti _______________ .01 nd .01 .01 nd nd .01 .01 .01 A1 _______________ 3.01 3.01 3.04 3.00 3.01 3.06 2.89 2.75 2.94 Fe _______________ 2.67 2.68 2.61 2.53 2.52 2.35 2.52 2.53 2.45 Mn ______________ .02 .02 .02 .01 .01 .00 .01 .00 .01 Mg ______________ 1.70 1.76 1.65 1.90 1.94 1.64 1.99 2.02. 1.95 Ca _______________ .00 .00 .00 .00 .00 .00 .00 .00 .00 Na _______________ .00 nd .00 .00 nd nd .00 .00 .00 K ________________ .04 nd .01 .01 nd nd .01 .01 .08 F ________________ nd nd nd nd nd nd nd nd nd Cation sum ___ 9.98 9.98 9.90 9.98 9.99 9.75 9.99 9.97 9.99 Notes Next to Next to Next to Next to Next to Mu St t 131: Bt 153001 172001. 172006 181006 181006 and and Mu 172002 181008 80 METAMORPHIC MINERAL ASSEMBLAGE-S, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 3.—Microprobe data on various minerals in samples of rooks from within and around the Taconic allochthon—Continued C. CHLORITE—Continued Sample ________________ 515-1 1052—2 1169—1 Spot __________________ 191014 192002 30 31 32 213012 215004 Weight percent Si_02 _____________ 22.98 22.6 24.86 24.91 24.31 23.73 24.52 T102 _____________ .41 .23 .16 .11 .09 .04 .04 A1203 _____________ 23.68 23.66 22.64 22.81 23.61 22.03 21.73 FeO ______________ 29.51 29.76 24.43 25.16 25.52 28.71 28.56 MnO _____________ .05 .04 .04 .02 .04 .44 .43 MgO ____________ — 10 99 10 92 14.83 14.64 13 74 11 91 12 54 CaO _____________ 02 03 .01 .00 02 02 05 Na20 _____________ 02 00 .01 .00 08 O3 01 2 ______________ 05 04 .00 .00 00 01 02 F _______________ - nd nd .04 .04 05 nd nd Subsum ______ 87 71 87 35 86.99 87.66 87 62 86 92 87 90 2 ______________ 11 10 11 03 11.36 11.42 11 35 11 02 11 19 Sum _______ 98 81 98 38 98 35 99.08 98 97 97 94 99 09 Number of atoms Si _______________ 2.48 2.46 2.62 2.62 2.57 2.58 2.63 Ti _______________ .03 .02 .01 .01 .01 .00 .00 A1 _______________ 3.01 3.03 2.81 2.82 2.94 2.83 2.75 Fe _______________ 2.67 2.71 2.16 2.21 2.25 2.61 2.56 Mn ______________ .00 .00 .00 .00 .00 .04 .04 Mg' ______________ 1.77 1.77 2.33 2.29 2.16 1.93 2.01 Ga _______________ .00 .00 .00 .00 .00 .00 .01 Na _______________ .00 .00 .00 .00 .02 .01 .00 K ________________ .01 .01 .010 .00 .00 .00 .00 F ________________ nd nd .01 .01 .02 nd nd Cation sum ___ 9.97 10.00 9.93 9.95 9.97 10.00 10.00 Notes Next to Next to Zn0=0.00 Zn0=0.00 Weight Next to 11m 11m percent Ep 191015 192001 sum in- 215002 and Mu eludes 191016 ZnO: 0.20; catior} sum m- eludes 0.02 atom Zn TABLES 81 TABLE 3.—-—Microprobe data on various minerals in samples of rocks from within and around the Taconic allochthonRContinued D. CHLORITOID Sample ________________ 45—1 103—1 103—2 Spot __________________ 1 3 051001 051002 061002 061005 061009 062006 062007 Weight percent S102 _____________ 23.76 23.38 23.74 23.78 23.62 24.06 23.59 23.29 23.69 Ti02 _____________ .00 .01 .06 nd .02 .04 nd nd nd A1203 _____________ 40.59 41.80 39.57 39.80 319.86 319.97 40.01 39.07 40.30 FeO ______________ 25.32 25.10 25.55 25.73 24.71 24.46 24.53 24.57 24.53 MnO _____________ 1.16 1.36 .29 .30 .40 .41 .43 .44 .38 MgO ____________ - 1.41 1.22 2.02 1.95 2.18 2.46 2.34 2.26 2.27 0210 _____________ .02 .02 .03 .03 .04 .01 .03 .07 .00 Na20 _____________ .00 .00 .05 nd .04 .03 nd nd nd K20 ______________ .16 .13 .05 nd .02 .00 nd nd nd F _______________ - nd nd nd nd nd nd nd nd nd Subsum ______ 92.42 93.02 91.36 91.59 90.89 91.44 90.93 89.70 91.17 H20 ______________ 7.18 7.23 7.11 7.13 7.10 7.17 7.12 7.00 7.14 Sum _______ 99.60 100.25 98.47 98.72 97.99 98.61 98.05 96.70 98.31 Number of atoms SI _______________ 0 99 0 97 1 00 1.00 1.00 1 01 0 99 1 00 1 00 T1 _______________ 00 00 00 nd .00 00 nd nd nd A1 _______________ 2 00 2 04 1 97 1.97 1.98 1 97 1 99 1 97 1 99 Fe _______________ 88 87 90 .91 .87 86 86 88 86 Mn ______________ 04 05 01 .01 .01 01 02 02 01 Mg‘ ______________ 09 08 13 .12 .14 15 15 14 14 Ca _______________ 00 00 00 .00 .00 00 00 0‘0 00 Na _______________ 00 00 00 nd .00 00 nd nd nd K ________________ 00 00 00 nd .00 00 nd nd nd F ________________ nd nd nd nd nd nd nd nd nd Cation sum ___ 4.00 4.01 4.01 4.01 4.00 4.00 4.01 4.01 4.00 Notes Average Average Next to Next to Next to In Ga In con- In con- In con- of 3 of 3 Bt Bt Bt 061006 tact tact tact deter- deter- 051005, 051007; 061001 with with with mina- mina- and in same B1; B1: Ch tions tions 051007; crystal 061010 062002 062009 in same as spot and crystal 051001 Bt asspot 062008 051002 82 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 3.—Mderom'obe data on various minerals in samples of rocks from within and around the Tacom'c allochthon—Continued D. CHLORITOID—Continued Sample ________________ 103—2—Continued 140'2 ______—2 34—1 Spot __________________ 06201 1 1 2 3 4 ‘ 15 16 Weight percent S102 _____________ 23.52 24.03 23.41 24.01 23.96 24.34 23.39 TiOa _____________ nd .09 .02 .00 .04 .00 .06 A1203 _____________ 39.65 40.15 41.14 40.36 41.46 40.38 40.73 FeO ______________ 24.78 24.33 24.06 23.61 23.87 24.75 23.96 MnO _____________ .36 1.36 1.36 1.19 1.15 .13 .03 MgO ____________ - 2.23 1.32 1.23 1.65 1.61 2.48 2.64 03.0 _____________ .03 .04 .05 .05 .02 .00 .04 Na20 _____________ nd nd nd nd nd .04 .01 K20 ______________ nd nd nd nd nd .00 .00 F _______________ — nd .00 nd .00 .00 nd .00 Subsum ______ 90.57 91.32 91.71 90.87 92.11 92.35 90.86 H20 ______________ 7.08 7.13 7.15 7.13 7.23 7.24 7.15 Sum _______ 97.65 98.45 98.86 98.00 99.34 99.59 98.01 Number of atoms S1 _______________ 1 00 1 01 0.98 1.01 0 99 1 01 0 98 T1 _______________ nd 00 .00 .00 00 00 00 A1 _______________ 1 98 1 99 2.03 2.00 2 03 1 97 2 01 Fe _______________ 88 85 .84 .83 83 86 84 Mn ______________ 01 05 .05 04 04 00 00 Mg ______________ 14 08 .08 10 10 15 17 Ca _______________ 00 00 .00 00 00 00 00 Na, _______________ nd nd nd nd nd 00 00 K ________________ nd nd nd nd nd 00 00 F ________________ nd .00 nd .00 .00 nd .00 Cation sum ___ 4.01 3.98 3.99 3.98 3.99 4.00 4.00 Notes In con- Zn0=0.00 Weight Zn0=0.00 Zn0:0.00 Weight Zn0:0.00 tact percent percent with Bt sum sum 062012 mcludes mcludes ZnO: Zn0_ 0.44; 0 23 cation cation sum sum mcludm includes 0.01 0 01 atom atom TABLES 83 TABLE 3.—M’icroprobe data on various minerals in samples of rocks from within and around the Tacom'c allochthon—Continued D. CHLORITOID—Continued Sample ________________ 331—1 Spot __________________ 091001 091003 092005 092006 092007 092010 092011 093006 Weight percent 23.91 24.15 24.32 23.91 23.80 24.31 23.91 24.19 .12 nd nd .03 nd .06 nd nd 39.95 40.03 40.75 40.61 40.73 40.69 40.70 40.18 24.89 24.92 25.08 24.62 24.84 23.95 24.57 24.73 .20 .18 .19 .20 .18 .63 .28 .17 2.38 2.49 2.38 2.36 2.39 2.63 2.39 2.19 .00 .00 nd .00 nd .00 nd .01 .03 nd mi .03 nd .03 nd nd .02 nd nd .02 nd .00 nd nd nd nd nd nd nd nd nd g 91.50 91.77 92.72 91.78 91.94 92.30 91.85 91.47 7.16 7.19 7.27 7.20 7.20 7.26 7.21 7.18 98.66 98.96 99.99 98.98 99.14 99.56 99.06 98.65 Number of atoms S1 _______________ 1 00 1 01 1.00 1.00 0 99 1 01 1 00 1 01 T1 _______________ 00 nd nd .00 n 0 nd nd Al _______________ 1 97 1 97 1.98 1.99 2 00 1 98 2 00 1 98 Fe _______________ 87 87 .87 .86 86 83 86 86 Mn ______________ 01 01 01 .01 01 02 01 01 Mg ______________ 15 16 15 .15 15 16 15 14 Ca _______________ 00 00 nd .00 nd 00 nd 00 Na _______________ 00 nd nd .00 nd 00 nd nd ________________ 00 nd nd .00 nd 00 nd nd F ________________ nd nd nd nd nd nd nd nd Cation sum ___ 4.00 4.02 4.01 4.01 4.01 4.00 4.02 4.00 Notes Next to Spots 092006 and In Ga In Ga Next to 11m 092007 are in St 091002 same crystal (318004. ' 093005. and Mn 84 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 3.—Mic1‘opro be data, on various minerals in samples of rocks from within and around the Tacom'e allochthon—Continued D. CHLORITOID%ontinued Sanufle ________________ 338—1 339-1 369—1 Spot .................. 101002 102005 102006 102007 102008 111005 111006 131001 131002 Weight percent 8102 _____________ 24.07 23.94 23.69 23.31 24.12 24.33 24.16 24.04 24.22 Ti02 _____________ .05 .06 nd nd nd .05 nd .02 nd A1203 ___________ -_ 40.37 40.48 40.01 40.17 40.24 40.72 40.57 40.53 40.78 FeO _____________ 25.12 25.61 25.62 25.75 25.57 24.37 24.45 24.17 24.47 MnO ____________ — .16 .23 .23 .24 .24 .35 .48 .99 1.12 MgO _____________ 2.01 1.99 2.05 2.00 1.97 2.65 2.60 2.15 2.17 CaO _____________ .00 .00 nd nd nd .02 .04 .00 nd Na20 __- __________ .07 .04 nd nd nd .04 nd .04 nd K20 ______________ .08 .03 nd nd nd .00 n-d .01 nd F ________________ nd nd nd nd nd nd nd nd nd Subsum ______ 91.93 92.38 91.60 91.47 92.14 92.53 92.30 91.95 92.76 H20 ______________ 7.19 7.21 7.14 7.12 7.19 7.27 7.24 7.20 7.26 Sum _______ 99.12 99.59 98.74 98.59 99.33 99.80 99.54 99.15 100.02 Number of atoms Si ________________ 1.00 1.00 1.00 0.98 1.01 1.00 1.00 1.00 1.00 T‘1 _______________ 00 .00 nd nd nd .00 nd .00 nd A1 _______________ 1 98 1 99 1 98 2.00 1.98 1 98 1 98 1 99 1 99 Fe _______________ 88 89 90 .91 .89 84 85 84 85 Mn ______________ 01 01 01 .01 01 01 02 04 04 Mg“ ______________ 12 12 13 .13 .12 16 16 13 13 Ca _______________ 00 00 nd nd nd 00 00 00 nd Na _______________ 00 00 nd nd nd 00 nd 00 nd K ________________ 00 00 nd nd nd 00 nd 00 nd F ________________ nd nd nd nd nd nd nd nd nd Cation sum ___ 3.99 4.01 4.02 4.03 4.01 3.99 4.01 4.00 4.01 Notes Next to Next to Next to On rim Next to In Ga In same In rim Next to B1: Ilm Be of Mu crystal Ga as of Ga Ga 101001 102001 102010; crystal 102009; contain- spot 131003; and Mu and Mu in same contain- in same ing 111005; in same 101003 102009; crystal ing crystal spots next to cryshm in same as spots spots as spots 111001— Ga as spot crystal 102005, 102005, 102005~ 111004, 111007 131001, as spots 102007 102006, 102007 in con- nearer 102006— and and tact rhn of 102008 102008 102008 with Ga. Ga 111004 TABLES 85 TABLE 3.—M2'c'rozwobe data on various minerals in samples of rocks from within and around the Tacom'c allochthon—Continued D. CHLORITOID—Continued Sample ________________ 369—1—Continued 463—1 466—1 Spot __________________ 131004 131010 141002 141003 141004 141007 141008 141024 151001 Weight percent Si02 _____________ 23.73 24.21 24.18 23.95 23.87 24.25 23.79 24.34 24.38 T102 _____________ nd nd .32 .09 nd nd nd nd .04 A1203 ___________ —_ 40.67 40.38 40.90 40.65 40.54 40.54 41.03 41.14 40.60 FeO _____________ 24.47 24.58 24.26 24.52 24.51 24.14 24.33 24.44 24.96 MnO ____________ — .99 1.05 .35 .38 .35 .33 .34 .36 .63 MgO _____________ 2.22 2.16 2.41 2.29 2.35 2.54 2.49 2.40 2.09 CaO _____________ .01 .01 .00 .00 nd nd nd nd .00 Na20 -_— __________ nd nd .04 .04 nd nd nd nd .08 K20 ______________ nd nd .04 .05 nd nd nd nd .03 ________________ nd nd nd nd nd nd nd nd nd Subsum ______ 92.09 92.39 92.50 91.97 91.62 91.80 91.98 92.68 92.81 H20 ______________ 7.20 7.22 7.27 7.21 7.18 7.22 7.22 7.29 7.26 Sum _______ 99.29 99.61 99.77 99.18 98.80 99.02 99.20 99.97 100.07 Number of atoms Si ________________ 0.99 1.01 1.00 1.00 1.00 1.01 0.99 1.00 1.00 ' nd nd 01 00 nd nd nd nd .00 2 00 1 98 1 99 1 99 1.99 1.99 2 01 2 00 1 98 85 84 85 .86 .84 85 84 86 04 04 01 01 .01 .01 01 01 02 14 13 15 14 .15 .16 15 15 13 00 00 00 00 nd nd nd nd 00 nd nd 00 00 nd nd nd nd 01 nd nd 00 00 nd nd nd nd 00 nd nd nd nd nd nd nd nd nd Cation sum ___ 4.02 4.01 4.00 3.99 4.01 4.01 4.01 4.00 4.00 Notes Center In Ga Near Bt In same On twin Next to In same Next to End of of Ga; 131011, 141005 crystal bound- Bt crystal u crystal next to near and as spots ary in 141006: as spots 141023 next to Ga rim next to 141002, same in same 141002, Mu 131005 Ilnl 141004, crystal Crystal 141003, 151002; 141001; 141007, as spots as spots 141004, in same in same and 141002, 141002, and crystal crystal 141008 141003, 141003, 141007 as spot as spots 141007, 141004, 151008 141003, and and 141004, 141008 141008 141007, and 141008 86 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 3.-——M1’croprobe data on various minerals in samples of rocks from within and around the Tacom'c allochthon—Continued D. CHLORITOID—Continued Sample ................ 466—1—Continued 506—1 509—1 Spot __________________ 151008 152001 172008 172009 172011 174005 181001 181005 182003 Weight percent 8102 _____________ 24.28 24.07 24.34 24.06 23.545 24.09 24.50 24.19 24.34 T102 _____________ nd .03 .06 nd nd .04 .04 .03 .02 A1203 ___________ -_ 40.29 40.70 40.03 40.23 40.30 39.79 40.05 40.48 40.64 FeO _____________ 25.11 25.13 24.92 25.09 24.97 25.02 24.81 24.68 24.26 MnO ____________ — .62 .73 .15 .15 .15 .15 .29 .22 .20 MgO _____________ 2.12 1.87 2.12 2.22 2.21 2.22 2.41 2.49 2.56 CaO _____________ .00 .00 .02 .02 .00 .02 .00 .01 .0‘0 Na20 __- __________ nd .03 .02 nd nd .04 .03 .04 .04 K20 ______________ nd .05 .05 nd nd .07 .06 .03 .05 F ________________ nd nd nd nd nd nd nd nd nd Subsum ______ 92.42 92.61 91.71 91.77 91.18 91.44 92.19 92.17 92.11 20 ______________ 7.22 7.23 7.18 7.18 7.13 7.15 7.22 7.23 7.24 Sum _______ 99.64 99.84 98.89 98.95 98.31 98.59 99.41 99.40 99.35 Number of atoms Si ________________ 1.01 1.00 1.02 1.00 0.99 1.01 1.02 1.00 1.01 Ti _______________ nd .00 .00 nd nd .00 .00 .00 .00 A1 _______________ 1.97 1.99 1.97 1.98 2.00 1.97 1.96 1.98 1.98 Fe _______________ .87 .87 .87 .88 .88 88 .86 .86 .84 Mn ______________ .02 03 01 .01 01 01 .01 01 01 Mg ______________ 13 12 13 .14 14 14 15 15 16 Ca _______________ 00 00 00 .00 00 00 00 00 00 Na _______________ nd 00 00 nd nd 00 00 00 01 K ________________ nd 00 00 nd nd 00 00 00 01 F ________________ nd nd nd nd nd nd nd nd nd Cation sum ___ 4.00 4.01 4.00 4.01 4.02 4.01 4.00 4.00 4.02 Notes In same Center Next to Next to Near Ga Next to Near B1: Next to crystal of St St 172013 Bt 181006 Bt as spot crysufl 172001 172001 and 181003, and 182005 151001 and Mn and Mu next to and Mu next to and Mu 172010 172010 Ga 181002 u 182004 172014 and 181008; 181008; in sarne in same crystal crystal as spot as spot 181001 181005 TABLES 87 TABLE 3.——Microprobe data on various minerals in samples of rocks from within and around the Tacom‘c allochthon—Continued D. CHLORITOID—Continued Sample ________________ 509-1—Continued 515-1 Spot __________________ 182008 191008 191010 191018 191021 191104 191105 191106 Weight percent SiO; _____________ 24.03 23.61 23.82 24.21 23.88 24.34 24.45 24.39 TiOa _____________ .05 .22 .01 .02 .03 nd nd nd A1203 ___________ -_ 40.47 40.16 40.93 40.73 40.29 40.31 40.10 40.69 FeO _____________ 24.13 25.11 25.03 25.61 25.43 26.02 26.07 25.67 MnO ____________ - .22 .36 .60 .14 .13 .13 .16 .14 MgO _____________ 2.48 2.41 2.49 2.14 2.20 1.82 1.95 2.20 09.0 _____________ .00 .06 .04 .01 .02 .07 .01 .02 Na20 __- __________ .03 .04 .03 .03 .04 nd nd nd K20 ______________ .01 .00 .01 .05 .01 nd nd nd ________________ nd nd nd nd nd nd nd nd Subsum ______ 91.42 91.97 92.96 92.94 92.03 92.69 92.74 93.11 H20 ______________ 7.19 7.18 7.26 7.26 7.19 7.23 7.23 7.28 Sum _______ 98.61 99.15 100.22 100.20 99.22 99.92 99.97 100.39 Number of atoms Si ________________ 1.00 0.99 0.98 1.00 1.00 1.01 1.01 1.01 Ti _______________ .00 .01 .00 .00 .00 nd nd nd A1 _______________ 1.99 1 98 1 99 1.98 1.98 1 97 1 96 1 98 Fe _______________ 84 88 84 .88 .89 90 90 88 Mn ______________ 01 01 02 .00 00 00 01 00 Mg ______________ 15 15 15 .13 14 11 12 13 Ca _______________ 00 00 00 .00 00 00 00 00 Na _______________ 00 00 00 .00 00 nd nd nd K ________________ 00 00 00 .00 00 nd nd nd F ________________ nd nd nd nd nd nd nd nd Cation sum ___ 3.99 4.02 3.98 3.99 4.01 3.99 4.00 4.00 Notes At rim of In Ga In Ga In con- Spots 191104, 191105, and 191106 Ga near contain- contain- tact are in same crystal spot Ga ing spot ing spot with St 182009 Ga Ga. 191019, 191004; 191004; in paral- next to near Ga 1e] 11m 191012 orien- 191009 tation 88 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 3.—Microprobe data on various minerals in samples of rocks from within and around the Taconic allochthon—Continued E. STAUROLITE Sample ________________ 14—1 234—1 Spot __________________ 1 2 3 4 5 1 2 9 Weight percent S102 _____________ 27.72 28.43 27.58 29.11 28.31 28.41 27.75 28.07 T102 _____________ .57 .75 .73 .42 .66 .76 .39 .58 A1203 ___________ -_ 52.88 53.72 52.78 53.37 53.73 51.55 52.00 53.21 FeO _____________ 14.04 13.38 14.18 14.33 14.31 14.36 14.13 12.89 MnO ____________ ~ .06 .02 .02 .‘08 .05 .00 .05 .12 MgO _____________ 1.53 1.37 1.37 1.43 1.37 1.05 .94 1.07 0210 _____________ .01 .01 .00 .010 .00 .00 .02 .05 Na20 __- __________ .00 .00 .00 .00 .00 .00 .00 nd K20 ______________ nd nd nd nd nd .00 .00 nd ________________ .00 .00 .00 .20 .08 .59 .26 nd Subsum ______ 97.11 97.88 97.10 98.91 98.64 96.79 96.23 95.99 H20 1 _____________ 1.05 1.06 1.05 1.07 1.07 1.04 1.03 1.04 Sum _______ 98.16 98.94 98.15 99.98 99.71 97.83 97.26 97.03 Number of atoms Si ________________ 3.97 4.01 3.96 4.07 3.98 4.09 4.02 4.03 Ti _______________ .06 .08 .08 .04 .07 .08 .04 .06 Al _______________ 8.92 8.94 8.92 8.79 8.91 8.75 8.89 9.01 Fe _______________ 1.68 1.58 1.70 1.67 1.68 1.73 1.71 1.55 Mn ______________ 01 00 00 .01 .01 .00 0‘0 .01 Mg ______________ 32 29 29 .30 29 22 20 23 Ca _______________ 02 00 00 .00 00 00 00 00 Na _______________ 00 00 00 .00 00 00 00 nd K ________________ nd nd nd nd nd 00 00 nd F ________________ .00 00 .00 .09 04 27 12 nd Cation sum ___ 15.01 14.92 15.00 14.89 14.96 14.93 14.96 14.89 Notes Weight Weight Weight Weight Weight Weight Weight perceut percent percept percept percept percent perceut sum In- sum m- sum “1- sum m- 511m m- sum m- sum m- cludes eludes eludes eludes eludes eludes eludes ZnO ZnO ZnO ZnO ZnO n ZnO 20.30; 20.20; 20.44; 20.14; 20.20; 20.57; 20.91; catior} catiou catiou cation? catiox} catiou catim} sum In- sum ll’l- sum m- sum m- sum “'1- sum m- sum In- cludes eludes eludes eludes cludes eludes eludes 0.03 0.02 0.05 0.01 0.02 0.06 0.10 atom atom atom atom atom atom atom Zn; Zn Zn Zn Zn Zn Zn average of 3 analyses See footnotes at end of table, p. 123. TABLES 89 TABLE 3.—Microprobe data on various minerals in samples of rocks from within and around the Tacom'c allochthon—Continued E. STAUROLITE—Continued Sample ________________ 290—1 331—1 Spot __________________ 14 15 16 091008 091009 091010 093004 093009 093010 Weight percent S-i02 ______________ 28.36 27.83 28.75 28.06 27.80 27.93 27.60 26.89 26.94 Ti02 _____________ .50 .51 .35 .52 .46 nd .58 .52 nd A1203 _____________ 52.81 53.34 53.45 54.78 54.91 54.45 54.00 54.10 54.64 FeO ______________ 13.61 12.78 12.96 13.53 13.38 13.26 13.76 13.47 13.33 MnO _____________ .07 .02 .01 .12 .11 .14 .15 .14 .14 MgO _____________ 1.31 1.49 1.59 1.00 .99 1.15 1.10 1.10 1.00 CaO ______________ .02 .01 .01 .00 .00 .00 .00 .00 .00 NaZO _____________ .02. .02 .07 .01 .00 nd .00 .00 nd K20 ______________ nd nd nd .01 .00 nd .00 .00 nd F ________________ .03 .09 .13 nd nd nd nd nd nd Sub‘sum ______ 97.13 96.24 97.55 98.03 97.65 96.93 97.19 96.22 96.05 H20 1 _____________ 1.05 1.05 1.06 1.06 1.06 1.05 1.05 1.04 1.04 Sum _______ 98.18 97.29 98.61 99.09 98.71 97.98 98.24 97.26 97.09 Number of atoms Si _______________ 4.05 3.99 4.07 3.96 3.93 3.98 3.94 3.87 3.88 Ti _______________ .05 .06 .04 .06 .05 nd .06 .06 nd Al _______________ 8.89 9.01 8.93 9.10 9.15 9.14 9.08 9.18 9.27 Fe _______________ 1 63 1 53 1 54 1.60 1.58 1 58 1 64 1 62 1 60 Mn ______________ 01 00 00 .01 .01 02 02 02 02 Mg ______________ 28 32 34 .21 .21 24 23 24 21 Ca _______________ 03 00 00 .00 .00 00 00 00 00 Na _______________ OO 00 00 .00 .00 nd 00 00 nd K ________________ nd nd nd .00 .00 nd 00 00 nd F ________________ .01 .04 01 nd nd nd nd nd nd Cation sum ..__ 14.99 14.93 14.95 14.94 14.93 14.96 14.97 14.99 14.98 Notes Weight Weight Weight Next to In same Next to percent percent percent Cd crystal Ch sum in- sum in- sum in- 093006, as spot 093011 eludes eludes eludes Ch 093004 Z110 Z110 ZnO 093005, =0.43; =0.23; 20.34; Bt cation cation cation 093003, sum in- sum in- sum in- and Mu eludes eludes eludes 093007; 0.05 0.02 0.03 in same atom atom atom crystal Zn; Zn; Zn; as spot average average average 093009 of 3 of 3 of 3 analyses analyses analyses See footnotes at end of table, 1). 123. 90 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS, CONN., N.Y. TABLE 3.——Micropro be data on various minerals in samples of rocks from within and around the Taconic allochthon—Continued E. STAUROLITE—Continued Sample ________________ 339—1 355-1 Spot __________________ 111015 111016 111018 111028 1 2 3 4 5 Weight percent 8102 ______________ 27.07 27.88 217.97 27.85 28.33 27.75 28.12 27.73 28.48 T102 _____________ .57 nd .49 nd .64 .60 .36 .77 .75 A1203 _____________ 53.74 53.99 53.72 53.51 54.79 54.52 53.31 54.64 55.71 FeO ______________ 15.17 15.09 14.96 14.88 14.25 13.96 14.23 14.04 13.80 MnO _____________ .09 .10 .07 .10 .07 .04 .02 .07 .07 MgO _____________ 1.17 1.40 1.34 1.34 1.07 1.05 1.05 1.07 .99 CaO ______________ .00 .02 .01 .02 .00 .00 .02 .00 .00 N320 _____________ .00 nd .00 nd nd nd nd nd nd K20 ______________ .02 nd .00 nd nd nd nd nd nd F ________________ nd nd nd nd nd nd nd nd nd Subsum ______ 97.83 98.48 98.56 97.70 99.15 98.49 97.44 99.09 99.89 H20 1 _____________ 1.05 1.06 1.06 1.05 1.07 1.06 1.05 1.07 1.08 Sum _______ 98.88 99.54 99.62 98.75 100.22 99.55 98.49 100.16 100.97 Number of atoms 3.87 3.95 3.95 3.97 3.96 3.92 4.01 3.90 3.94 .06 nd .05 nd .06 .06 .04 .08 .08 9 05 9 01 8 95 8 99 9.03 9 07 8 96 9 05 9.09 1 81 1 79 1 77 1 77 1.67 1 65 1 70 1 65 1 60 01 01 01 01 .00 00 00 01 01 25 30 28 28 .22 22 22 22 20 00 00 00 00 .00 00 00 00 00 00 nd 00 nd nd nd nd nd nd 00 nd 00 nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd 15.05 15.06 15.01 15.02 14.94 14.98 14.96 14.99 14.93 Notes Next to In same Next to Zn0=0.00 Weight Weight Weight Weight Ga crystal Mu Samples percent percent percent percent 111014; as spot 111029 1—5, in- sum in- sum in- sum in- sum in- in same 111015 elusive, eludes eludes eludes eludes crystal used in ZnO ZnO ZnO 2110 as spot compar- 20.57; 20.33; =0.77; =0.09; 111016 ison cation cation cation cation with sum in- sum in- sum in- sum in- wet- cludes eludes eludes eludes chemical 0.06 0.03 0.08 0.01 analysis atom atom atom atom 5(fable Zn Zn Zn Zn prébe done on aliquot of min- eral sep- arate See footnobes at end of table, 1). 123. TABLES 91 TABLE 3.—Microprobe data on various minerals in samples of rocks from within and around the Taconic allochthon—Continued E. V STAUROLITE—Continued Sample ________________ 355-1—Continued 356—1 369-1 Spot .................. 17 18 20 21 22 23 24 132001 132002 Weight percent SiOz ______________ 27.40 28.07 28.73 27.97 27.97 28.87 28.05 27.53 27.51 Ti02 _____________ .76 .71 .65 .50 .52 .62 .63 .54 nd A1203 _____________ 51.14 53.63 53.91 54.11 53.50 53.74 52.51 54.46 54.59 FeO ______________ 14.29 14.46 13.96 13.30 13.66 13.74 13.28 12.87 12.66 MnO _____________ .04 .09 .05 .04 .05 .02 .10 .11 .07 Mg'O _____________ 1.02 .99 1.52 1.52 1.59 1.43 1.46 .86 .81 CaO ______________ .00 .00 .01 .01 .00 .00 .03 .00 .00 No.20 _____________ .00 .00 .00 .00 .00 .00 .00 .04 nd 2 ______________ .00 .00 nd nd nd nd nd .00 nd ________________ .00 .00 .00 .26 .10 .00 .00 nd nd Sub-sum ______ 94.83 98.02 99.16 97.95 97.46 98.77 96.16 96.41 ' 95.64 H20 ‘1 _____________ 1.02 1.06 1.07 1.06 1.06 1.06 1.04 1.05 1.04 Sum _______ 95.85 99.08 100.23 99.01 98.52 99.83 97.20 97.46 96.68 Number of atoms Si _______________ 4.03 3.98 4.02 3.94 3.97 3.96 4.04 3.94 3.96 Ti _______________ .08 .08 .07 .05 .06 .07 .07 .06 nd A1 _______________ 8.85 8.96 8.90 9.00 8.96 9.00 8.91 9.18 9.25 Fe _______________ 1 76 1 71 1 64 1.57 1.62 1 64 1 60 1 54 1 52 Mn ______________ 00 01 01 .00 .01 00 01 01 01 Mg ______________ 22 21 32 .32 .34 30 31 18 17 Ca _______________ 00 00 00 .00 .00 00 00 00 00 Na _______________ 00 00 00 .00 .00 00 00 01 nd K ________________ 00 00 nd nd nd nd nd 00 nd F ________________ .00 .00 .00 .12 .05 .00 00 nd nd Cation sum ___ 14.96 14.96 14.99 14.93 14.98 15.01 14.95 14.92 14.91 Notes Weight Weight Weight Weight Weight Weight Weight In same 0n rim of percent percent percent percent percent percent percent crystal crystal sum in- sum in- sum in- sum in- sum in- sum in- sum in- as spot contain- cludes cludes eludes eludes eludes cluda eludes 132002 ing spot ZnO ZnO ZnO Z110 Z110 Z110 ZnO 132001; =0.18; =0.07; 20.33; =0.46; =0.15; :0.36; 20.10; next to cation cation cation cation cation cation cation sum in- sum in- sum in- sum in- sum in- sum in- sum in- 132003 eludes eludes eludes cludes cludes eludes eludes . 0.01 0.03 0.05 0.02 0.04 0.01 atom atom atom atom atom atom atom Zn Zn Zn Zn Zn Zn Zn; average of 2 analyses See footnotes at end of table. p. 123. 92 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 3.—Microprobe data. on various minerals in samples of rocks from within and around the Tacom'c allochthon—Continued E. STAUROLITE—Continued Sample ________________ 463—1 506—1 515-1 Spot __________________ 142010 142011 1 2 172006 191019 191020 191022 Weight percent SiOa ______________ 27.09 26.99 29.15 28.66 27.52 28.00 27.73 27.94 TiOZ _____________ .43 nd .55 .61 nd .38 .40 .37 A1203 _____________ 53.18 53.58 53.52 53.75 53.48 54.32 54.35 54.48 FeO ______________ 15.60 15.50 11.78 11.76 12.71 11.92 11.83 11.98 MnO _____________ .24 .23 .10 .05 .08 .09 .08 .08 MgO _____________ 1.44 1.30 .94 .88 .88 .72 .71 .73 CaO ______________ .00 .00 .00 .00 .01 .00 .01 .01 Na20 _____________ .00 nd .00 .00 nd .06 .07 .08 20 ______________ .01 nd nd nd nd .04 .02 .02 F ________________ nd nd .30 .00 nd nd nd nd Subsum ______ 97.99 97.60 96.50 96.08 94.68 95.53 95.20 95.69 H2 _____________ 1.05 1.05 1.06 1.05 1.03 1.04 1.04 1.05 Sum _______ 99.04 98.65 97.56 97.13 95.71 96.57 96.24 96.74 Number of atoms Si ________________ 3.88 3.87 4.13 4.09 4.00 4.02 3.99 4.00 Ti _______________ .05 nd .06 .07 nd .04 .04 .04 A1 _______________ 8.97 9.06 8.93 9.05 9.17 9.19 9.22 9.20 Fe _______________ 1.87 1.86 1.39 1.41 1.55 1.43 1.42 1.43 Mn ______________ .03 .03 .01 .01 .01 .01 .01 .01 M2: ______________ .31 .28 .20 .19 .19 .15 .15 .16 Ca _______________ .00 .00 .00 .00 .00 .00 .00 .00 Na. _______________ .00 nd .00 .00 nd .02 .02 .02 K ________________ .00 nd nd nd nd .01 .00 .00 F ________________ nd nd .13 .00 nd nd nd nd Cation sum ___ 15.11 15.10 14.76 14.86 14.92 14.87 14.85 14.86 Notes On rim of In same Weight Weight Next to Next to In same In contact crystal crystal percent percent Ch Cd crystal with contain- as spot sum in- sum in- 172005 101018; as spots Cd; in ing spot 142010 eludes eludes in same 191019 same 142011 ZnO ZnO crystal and crystal =0.41; 20.37; as spots 191022 as spots cation cation 191020 191019 sum in- sum in- and and eludes eludes 191022 191020 0.04 0.04 atom atom Zn; Zn; average average of 2 of 3 analyses analyses See footnotes at end of table, p. 123. TABLES 93 TABLE 3.—Microprobe data on various minerals in samples of rocks from within and around the Taconic allochthon—Con-tinued E. STAUROLITE—Continued Sample ________________ 515—1—Continued 655—1 1052—2 Spot __________________ 192004 192005 192006 192007 1 2 3 4 16 Weight percent SiO2 ______________ 27.93 27.05 27.38 27.64 27.84 28.05 27.66 27.71 27.55 Ti02 _____________ .60 .47 .48 .46 .76 .69 .73 .64 .63 A1203 _____________ 54.38 54.12 54.45 54.57 50.91 52.79 52.72 53.12 51.88 FeO ______________ 11.65 11.99 12.03 11.52 14.89 14.85 14.68 14.53 15.08 MnO _____________ .10 .13 .11 .12 .05 .07 .07 .05 .02 MgO _____________ .75 .81 .72 .78 1.41 1.45 1.46 1.41 1.52 CIaO ______________ .01 .00 .02 .0‘0 .00 .04 .07 .0'0 .00 Na20 ______ __ ______ .09 .08 .10 .06 nd nd nd nd nd K20 ______________ .02 .01 .02 .03 nd nd nd nd nd ________________ nd nd nd nd nd nd nd nd nd Subsum ______ 95.53 94.66 95.31 95.18 95.86 97.94 97.39 97.46 96.68 H20 1 _____________ 1.05 1.03 1.04 1.04 1.03 1.05 1.05 1.05 1.04 Sum _______ 96.58 95.69 96.35 96.22 96.89 98.99 98.44 98.51 97.72 Number of atoms S1 ________________ 4.00 3.93 3.95 3.97 4.05 3.99 3.96 3.96 3.98 T1 _______________ .06 .05 .05 .05 .08 .07 .08 .07 .07 A1 _______________ 9.19 9.26 9.25 9.25 8.73 8.85 8.90 8.94 8.83 Fe _______________ 1.40 1.45 1.45 1.38 1.81 1.77 1.76 1.73 1.82 Mn ______________ .01 .02 .01 .01 .00 .01 .01 .00 .00 Mg“ ______________ 16 .18 .15 .17 .30 .30 .31 .30 .33 Ca _______________ .00 .00 .00 .00 .00 .00 .01 .00 .00 Na _______________ .03 .02 .03 .02 nd nd nd nd nd K ________________ .00 .00 .0'0 .01 nd nd nd nd nd F ________________ nd nd nd nd nd nd nd nd nd Cation sum ___ 14.85 14.91 14.89 14.86 14.97 14.99 15.03 15.00 15.03 Notes Spots 192004 and 192005 are in same crystal See footnotes at end of table, p. 123. 94 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 3.—Microprobe data on various minerals in samples of rocks from within and around the Taconic allochthon—Continued E. STAUROLITE—Continued Sample ________________ 1 052-2—Continued Spot __________________ 17 18 19 20 21 22 Weight percent 8102 ______________ 27.73 27.66 27.37 28.52 27.30 28.64 Ti02 _____________ .47 .97 .77 .71 .74 .86 A1203 _____________ 52.86 52.86 53.21 50.76 53.02 53.38 FeO ______________ 15.04 15.85 14.44 14.59 15.36 14.91 MnO _____________ .07 .02 .04 .03 .04 .09 MgO _____________ 1.56 1.66 1.57 1.29 1.23 1.33 CaO ______________ .00 .03 .03 .03 .00 .01 NaqO _____________ nd .00 .00 .00 .02 .00 KO ______________ nd .00 .00 .00 .00 .60 F ________________ nd .00 .13 .00 .23 .32 Subsum ______ 97.73 99.12 97.55 96.16 98.05 99.66 H20 1 _____________ 1.05 1.06 1.05 1.04 1.05 1.07 Sum _______ 98.78 100.18 98.60 97.20 99.10 100.73 Number of atoms Si ________________ 3.95 3.91 3.90 4.13 3.90 4.01 Ti _______________ .05 .10 .08 .08 .08 .09 A1 _______________ 8.89 8.82 8.95 8.66 8.93 8.81 Fe _______________ 1.79 1.87 1.72 1.76 1.83 1.74 Mn ______________ .00 .00 .00 .00 .00 .01 Mg ______________ .33 .35 .33 .28 .26 .28 Ca _______________ .00 .0‘0 .00 .00 .00 .00 Na _______________ nd .00 .00 .00 .00 .00 K ________________ nd .00 .00 .00 .00 .00 F ________________ nd .00 .06 .06 .10 .14 Cation sum ___ 15.01 15.05 14.99 14.93 15.03 14.98 Notes Weight Weight Weight Weight Weight pereept pereept percept pereept percent sum 1!]- sum In- sum 1n- sum In- sum m- cludes eludes eludes eludes eludes Z110 Z110 Zn.0 ZnO ZnO 20.07 20.10; 20.23; :0.30; =0.39: eatiot} eatiop eatiop eatiop sum m- sum In- sum In- sum in- cludes eludes eludes eludes 0.01 0.02 0.03 0.04 atom atom atom atom Zn Zn Zn Zn See footnotes at end of table, p. 123. TABLES 95 TABLE 3,—Microprobe data on various minerals in samples of rocks from within and around the Tacom'c allochtho'n—Continued F. GARNET 14—1 102—1 2 10 11 12 13 22 041001 041003 041004 Weight percent 38.57 36.15 34.98 34.16 36.89 35.38 36.49 36.33 36.60 .01 .02 .20 .00 .05 .00 .04 nd nd 20.42 20.52 21.17 21.32 20.93 20.66 21.13 20.73 21.13 38.48 37.35 36.23 36.49 36.96 38.23 28.06 28.34 29.33 .60 .84 .63 .98 1.11 .63 4.44 6.89 4.55 1.94 2.22 2.18 2.01 2.29 1.87 1.41 1.16 1.52 1.11 1.84 2.09 2.43 2.21 1.53 8.68 7.02 8.12 .05 nd .04 .04 nd .08 .00 nd nd .00 nd .00 .00 nd .00 .00 nd nd nd nd nd nd nd nd nd nd nd 101.18 99.19 98.81 98.47 100.47 99.53 100.25 100.47 101.25 Number of atoms 3.08 2.97 2.90 2.85 2.98 2.92 2.94 2.94 2.93 .00 .02 .01 .00 .00 .00 .00 nd nd 1.93 1.99 2.06 2.10 1.99 2.01 2.01 1.98 1.99 2.57 2.57 2.51 2.55 2.50 2.64 1.89 1.92 1.96 .04 .06 .04 .07 .08 .04 .30 .47 .31 .23 .27 .27 .25 .28 .23 .17 .14 .18 .09 .16 .19 .22 .19 .13 .75 .61 .70 .01 nd .00 .01 nd .01 .00 nd nd .00 nd .00 .00 nd .00 .00 nd nd nd nd nd nd nd nd nd nd nd Cation sum ___ 7.95 8.05 8.06 8.11 8.02 8.05 8.06 8.06 8.07 Notes Zn0:0.00 Rim of Part way Further Core of Near Ga In same 0n rim of Center of crystal into into crystal 10; crystal crystal crystal contain- crystal crystal contain- weight as spots contain- contain- ing contain- contain- ing percent 014003 ing ing spots 11 ing in: spots 10— sum in- and spots spots and 12; spots 10 spots 10 12; eludes 041004 041001 041001 weight and 12; and 11; weight ZnO and and percent weight weight percent =1.15; 041004 041003 sum in- percent percent 311mm. cation eludes sum in- sum in- cludes sum in- ZnO eludes eludes Z110 eludes =0.25; ZnO ZnO =o.03 0.07 cation :1.29; =1.04; atom sum in- cation cation Zn eludes sum in- sum in- 0.01 eludes eludes atom 0.08 0.06 Zn atom atom Zn Zn 96 METAMORPHIC MINERAL ASSEMBLAGE‘S, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 3.—Microprobe data on various minerals in samples of rocks from within and around the Taconic allochthon—Continued F. GARNET—Continued Sample ________________ 1 02«1——Continued 1 03—1 1 03—2 Spot ___________________ 043008 043009 052001 052014 052015 052016 061004 061006 061020 Weight percent 8102 ______________ 36.60 36.95 36.19 36.33 36.16 36.95 36.20 36.51 36.16 T102 _____________ .38 nd .00 nd nd .24 .01 .10 nd A1203 _____________ 20.81 21.18 20.96 210.95 20.73 20.81 20.79 20.64 20.44 FeO ______________ 29.34 29.23 36.87 37.13 36.06 35.10 34.09 33.87 34.43 MnO _____________ 4.43 4.67 3.03 3.16 4.00 4.34 3.97 4.10 4.24 MgO _____________ 1.27 1.34 1.37 1.14 1.37 1.45 1.47 1.58 1.38 CaO ______________ 7.88 7.93 1.47 1.34 1.68 1.76 3.01 3.15 2.65 NaZO _____________ .01 nd .01 nd nd .01 .02 .01 nd K20 ______________ .00 nd .05 nd nd .04 .04 .01 nd F ________________ nd nd nd nd nd nd nd nd nd Sum _______ 100.72 101.30 99.95 100.05 100.00 100.70 99.60 99.97 99.30 Number of atoms 2.94 2.95 2.96 2.97 2.96 2.99 2.97 2.97 2.98 .02 nd .00 nd nd .01 .00 .01 nd 1 97 1 99 2 02 2 02 2.00 1 99 2 01 1 98 1 98 1 97 1 95 2 53 2 54 2.47 2 38 2 34 2 31 2 37 30 32 21 22 .28 30 28 28 30 15 16 17 14 .17 17 18 19 17 68 68 13 12 .15 15 26 27 23 nd nd 00 nd nd 00 00 00 nd nd nd 00 nd nd 00 00 00 nd nd nd nd nd nd nd nd nd nd Cation sum ___ 8.03 8.05 8.02 8.01 8.03 7.99 8.04 8.01 8.03 Notes Center of Rim of Near 11m Rim of Part way Near core Contact Encloses crystal crystal 052003; crystal into of with Cd Cd contain- contain- in same contain- crystal crystal 061003 061005 ing ing crystal ing oontain- contain- spot spot as spots spots ing ing 043009 043008 052014, 052001, spots spots 052015, 052015. 052001, 052001, and and 052014, 052014, 052016 052016 d an and 052016 052015 TABLES 97 TABLE 3.—Microprobe data on various minerals in samples of rocks from within and around the Tacom'c allochthon—Continued F. GARNET—Continued Sample ________________ 161—1 234—1 289—2 Spot ___________________ 1 2 3 7 8 082001 082002 082003 082005 Weight percent Si02 ______________ 38.01 37.45 38.60 38.25 37.20 36.39 36.75 36.66 37.23 T102 _____________ .00 .09 .20 .00 .08 .26 .20 nd .04 A1203 ____________ 20.31 20.38 20.82 18.99 21.38 20.46 20.84 20.80 21.00 FeO _____________ 29.81 31.68 28.02 38.22 36.65 27.28 27.18 27.08 27.77 MnO _____________ 5.73 4.70 5.25 1.23 1.89 6.55 7.315 6.93 4.95 Mg'O _____________ 1.20 1.20 1.22 1.99 1.66 1.67 1.67 1.73 2.09 CaO ______________ 4.15 5.93 5.82 1.99 1.89 7.210 6.42 7.07 7.62 N320 _____________ .03 .04 .04 .02 nd .01 .01 nd .01 20 ______________ .00 .00 .00 .00 nd .00 .00 nd .00 ________________ nd nd nd ml nd nd nd nd nd Sum _______ 99.24 101.47 99.97 100.92 100.75 99.82 100.42 100.27 100.71 Number of atoms Si ________________ 3.08 3.00 3.08 3.09 3.00 2.95 2.96 2.96 2.97 ' .00 .00 .01 .00 .00 .02 .01 nd .00 1.94 1.92 1.96 1.81 2.03 1.96 1.98 1.98 1.98 2.02 2.12 1.87 2.58 2.47 1.85 1.83 1.83 1.85 .39 .32 .35 .08 .13 .45 .50 .47 .33 .14 .14 .15 .24 .20 .20 .20 .21 .25 .36 .51 .50 .17 .16 .63 .55 .61 .65 _0.0 .00 .00 .00 nd .00 .00 nd .00 .00 .00 .00 .00 nd .00 .00 nd .00 nd nd nd nd nd nd nd nd nd Cation sum ___ 7.93 8.01 7.92 7.98 7.99 8.06 8.03 8.06 8.03 Notes Weight Center of In same Near rim On rim of percent crystal crystal of crystal sum in- contain- as spots crystal contain- cludes ing 082001, contain- in: 2:10 spots 082003, ing spots = 0.23; 082002, and spots 082001, cation 082003, 082005 082001, 082002. sum in- and 082002, and eludes 082005 and 082003 0.01 082005 atom Zn 98 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 3.—Mic’ro;m‘obe data on various minerals in samples of rocks from within and around the Tacom'c allochthon—Continued F. GARNET—Continued Sample ________________ 290—1 331—1 Spot ___________________ 11 1 2 3 5 6 7 Weight percent Si02 ______________ 36.60 36.93 36.63 37.26 35.71 36.00 36.43 36.77 T102 _____________ .05 .04 .00 .00 .04 .00 .12 .00 A1203 ____________ 20.91 21.13 20.91 20.50 20.81 20.54 21.28 21.08 FeO _____________ 36.43 35.11 37.03 35.89 31.75 36.78 31.62 36.41 MnO _____________ .91 3.89 2.10 3.06 6.45 2.09 7.06 2.16 MgO _____________ 1.40 1.84 1.61 1.79 1.59 1.51 1.54 1.66 030 ______________ 2.51 1.48 1.39 1.47 1.75 1.43 1.85 1.51 Na20 _____________ .01 nd nd nd nd nd nd nd 20 ______________ .00 nd nd nd nd nd nd nd ________________ nd nd nd nd nd nd nd nd Sum _______ 98.82 100.42 99.67 99.97 98.10 98.35 99.90 99.59 Number of atoms Si ________________ 3.00 2.99 2.99 3.03 2.96 2.99 2.96 3.00 Ti _______________ .00 .00 .0'0 .00 .00 .00 .01 .00 Al _______________ 2.02 2.01 2.01 1.96 2.04 2.01 2.04 2.02 Fe _______________ 2.540 2.38 2.53 2.44 2.20 2.55 2.15 2.48 Mn ______________ .06 .27 .14 .21 .45 .15 .48 .15 Mg _______________ .17 .22 .19 .21 .19 .19 .19 .20 Ca _______________ .22 .13 .12 .13 .15 .13 .16 .13 Na _______________ .00 nd nd nd nd nd nd nd K ________________ .00 nd nd nd nd nd nd nd F ________________ nd nd nd nd nd nd nd nd Cation sum ___ 7.97 8.00 7.98 7.98 7.99 8.02 7.99 7.98 Notes Core of Rim of Near the Core of Rim of Core of Rim of crystal crystal rim of crystal crystal crystal crystal contain- contain- crystal contain- contain- contain- contain- ing ing contain- ing ing ing ing spots 2 spots 1 ing spots 5, spots 4. spots 4, spots 4, and 3 and 3 spots 1 6, and 7 6, and 7 5, and 7 5. and 6 and 2 TABLES 99 TABLE 3.—M2'croprobe data on various minerals in samples of rocks from within and around the Tacom'c allochthon—Continued F. GARNET—Continued Sample ________________ 331—1—Continued 339-1 355—1 Spot ___________________ 8 9 111002 111004 111007 111022 1 2 Weight percent SiO2 ______________ 37.34 36.46 37.47 37.42 37.65 36.97 36.29 36.81 T102 _____________ .03 .00 nd .02 .06 nd .25 .11 A1203 ____________ 20.29 20.14 21.11 21.25 21.12 21.38 21.24 21.18 FeO _____________ 35.26 37.20 37.83 34.03 35.13 37.94 36.90 36.03 .MnO _____________ 3.34 2.05 .95 4.25 3.02 1.06 1.34 2.04 MgO _____________ 1.82 1.78 1.90 1.66 1.85 1.97 1.70 1.95 0:10 ______________ 1.42 1.37 1.50 3.04 2.76 2.21 1.51 1.72 Nazo _____________ nd nd nd .00 .03 nd .01 .00 KO ______________ nd nd nd .01 .01 nd .00 .00 F ________________ nd nd nd nd nd nd nd nd Sum _______ 99.50 99.00 100.76 101.68 101.63 101.53 99.24 99.84 Number of atoms 3.05 3.01 3.02 2.99 3.00 2.96 2.97 2.99 .00 .00 nd .00 .00 nd .01 .00 1.95 1.96 2.00 2.00 1.99 2.02 2.05 2.03 2 40 2 57 2 55 2.27 2.34 2 54 2 52 2.45 23 14 06 29 .20 07 09 14 22 22 23 .20 .22 24 21 24 12 12 13 .26 .24 19 13 15 nd nd nd .00 .00 nd 00 00 nd nd nd .00 .00 nd 00 00 nd nd nd nd nd nd nd nd Cation sum ___ 7.97 8.02 7.99 8.01 7.99 8.02 7.98 8.00 Notes Core of Rim of On rim of In core of Near Cd Next to Zn0=0.00 ZnO: 0.00 crystal crystal crystal crystal 1 1 1 006; Ilm contain- contain- contain- contain- in same 111021— ing ing ing ing crystal 111023 spot 9 spot 8 spots spots as spots 111004 111002 111002 and and and 111007 111007; 111004 next to Cd 11100L Bt 111010 and Ch 111012 and 111013 100 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 3.—Mic’rom'obe data on various minerals in samples of rocks from within and around the Tacom'c allochthon—Continued F. GARNET—Continued 355—1—-Continued 356-1 4 5 1 3 9 1 1 12 14 Weight percent 36.48 36.96 36.83 37.12 36.22 36.68 36.93 36.20 .68 .17 .02 .09 .30 .09 .03 .11 20.60 21.66 21.25 19.53 21.37 19.68 20.54 21.39 33.91 34.64 36.88 37.63 33.58 34.99 37.14 36.73 2.84 3.29 .50 .58 3.93 3.07 1.50 .65 1.80 1.86 2.41 2.07 1.10 1.17 1.61 1.99 2.02 2.03 2.09 2.62 3.09 3.50 3.03 2.91 .00 .02 .00 nd .00 nd .00 nd .00 .00 .00 nd .00 nd .00 nd nd nd nd nd nd nd nd nd 98.33 100.63 100.25 99.66 99.59 99.25 100.78 100.28 Number of atoms 3.00 2.97 2.97 3.04 2.96 3.02 2.99 2.93 .04 .01 .00 .00 .02 .00 .00 .00 1.99 2.05 2.02 1.88 2.05 1.91 1.96 2.04 2.33 2.33 2.49 2.57 2.29 2.41 2.52 2.49 .20 .22 .03 .04 .27 .21 .10 .04 .22 .22 .29 .25 .13 .14 .19 .24 .18 .17 .18 .23 .27 .31 .26 .25 .00 .00 .00 nd .00 nd .00 nd .00 .00 .00 nd .00 nd .00 nd nd nd nd nd nd nd nd nd Cation sum ___ 7.96 7.97 7.99 8.01 7.99 8.00 8.02 8.01 Notes ZnO : 0.00 ZnO : 0.00 Weight Weight ZnO: 0. 00 Weight ZnO: 0. 00 Weight percept percept percept percept sum m- sum m- sum m- sum m- eludes eludes eludes eludes ZnO ZnO ZnO ZnO 20.27; 20.02 :0.07 20.30; catior} catiou sum ln- sum m- cludw eludes 0.01 0.02 atom atom Zn Zn TABLES 101 TABLE 3.—Microprobe data on various minerals in samples of rocks from within and around the Tacom'c allochthon—Continued F. GARNET—Continued Sample ________________ 3‘56—1—Continued 369—1 463—1 Spot ___________________ 15 42 46 131003 131005 131007 131011 141013 Weight percent Si02 ______________ 36.48 36.32 36.24 37.35 36.59 36.92 36.05 36.80 Ti02 _____________ .06 .20 .05 .10 nd .02 nd .01 A120a ____________ 21.27 20.65 21.27 20.95 21.16 21.28 21.39 19.79 FeO _____________ 37.50 35.62 37.72 30.05 29.04 36.30 29.28 36.30 MnO _____________ .75 2.52 .60 9.77 8.83 2.82 9.73 3.32 MgO _____________ 2.20 1.510 2.16 1.12 1.11 1.79 1.11 1.85 03.0 ______________ 1.80 3.37 2.85 2.94 3.30 2.48 3.05 1.37 Nazo _____________ nd nd nd .01 nd .01 nd .04 K20 ______________ nd nd nd .01 nd .01 nd .01 F ________________ nd nd nd nd nd nd nd nd Sum _______ 100.44 100.42 101.04 102.30 100.03 101.63 100.61 99.49 Number of atoms Si ________________ 2.95 2.95 2.92 2.99 2.98 2.96 2.93 3.02 Ti _______________ .00 .01 .00 .01 nd .00 nd .00 Al _______________ 2.03 1.98 2.02 1.9 2.03 2.01 2.05 1.92 Fe _______________ 2.54 2.42 2.54 2.01 1.98 2.44 1.99 2.49 Mn 05 .17 04 .66 .61 19 67 .23 26 18 26 .13 .13 21 13 23 15 29 25 .25 .29 21 27 12 nd nd nd .00 nd 00 nd 01 nd nd nd .00 nd 00 nd 00 nd nd nd nd nd nd nd nd Cation sum ___ 8.00 8.01 8.04 8.02 8.02 8.02 8.04 8.02 Notes Weight Weight Weight Next to Next to On rim: Near rim; On rim of percent percent percent Cd Cd next to nextto crystal sum in- sum in- sum in- 131002 131004 Ch Cd contain- cludes eludes eludes and near and at 131006 131010 in: ZnO ZnO ZnO rim of core spots :0.38; :0.24; =0.15; crystal 141014 cation cation cation an sum in- sum in- sum in- 141015 eludes cludes cludes 0.02 0.01 0.01 atom atom atom Zn Zn Zn 102 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 3.——Microprobe data on various minerals in samples of rocks from within and around the Tacom'c allochthon—-Continued F. GARNET—Continued Sample ________________ 463—1—Continued 487—2—4 506—1 Spot ___________________ 141014 141015 1 3 4 172013 172014 172015 172016 Weight percent S102 _____________ 36.59 36.58 36.59 35.70 36.62 35.86 36.48 36.28 36.64 T102 _____________ nd nd .08 .12 .11 .02 nd nd .05 A1203 _____________ 21.38 21.06 20.93 20.32 20.40 20.89 21.26 21.06 21.13 FeO ______________ 35.93 36.48 40.75 37.25 33.01 37.88 37.80 35.71 35.11 MnO _____________ 3.49 3.96 1.32 4.26 7.68 1.66 1.46 3.44 4.10 MgO _____________ 1.95 1.59 1.0-9 .84 .60 1.84 1.73 1.69 1.55 03.0 ______________ 1.24 1.19 .37 .74 2.58 1.65 1.76 1.73 1.60 N340 _____________ nd nd nd nd nd .01 nd nd .02 K20 ______________ nd nd nd nd nd .00 nd nd .01 F ________________ nd nd nd nd nd nd nd nd nd Sum _______ 100.58 100.86 101.15 99.89 101.00 99.81 100.49 99.91 100.21 Number of atoms Si ________________ 2.96 2.97 2.98 2.96 2.99 2.94 2.96 2.96 2.98 Ti ________________ nd nd .00 .01 .00 .00 nd nd .00 A1 _______________ 2.04 2.01 2.00 1.98 1.96 2.02 2.03 2.03 2.02 Fe _______________ 2.43 2.48 2.77 2.58 2.25 2.60 2.57 2.44 2.3.9 Mn ______________ .24 .27 .09 .30 .53 .12 .10 .24 .28 Mg ______________ .24 .19 .13 .10 .07 .22 .21 .21 .19 Ca _______________ .11 .10 .03 .06 .23 .15 .15 .15 .14 Na. _______________ nd nd nd nd nd .00 nd nd .00 K ________________ nd nd nd nd nd .00 nd nd .00 F ________________ nd nd nd nd nd nd nd nd nd Cation sum ___ 8.02 8.02 8.00 8.03 8.03 8.05 8.02 8.03 8.00 Notes Near rim Near Bt 0n rim of Most of Core of On rim, Nextto Farther Farther of 141005: crystal way crystal next to Cd into into crystal on rim contain- howald contain- Mu 172011; crystal crystal contain. of his core Of his 172012 on rim contain- contain- ing crystal spots 3 crystal spots 1 of ing ing spots contain- and 4: contain- and 3: crystal spots spots 141013 ing weight in: ZnO = 0 contain- 172014 172014 and spots percent spots 1 ing and. and 141015 141013 sum in— and 4; spots 172016 172015 and eludes weight 172015 than than 141014 Z‘nO perce.nt and spot spot =0,02 sum 111- 172016 172014 172015 eludes ZnO =0.66: cation sum m- dudes 0 ()4 atom Zn TABLES 103 TABLE 3.—Microprobe data on various minerals in samples of rocks from within and around the Taconic allochthon—Continued F. GARNET—Continued Spot ___________________ 182009 182010 182011 191004 191005 191007 191012 Weight percent SiOa _____________ 36.73 37.28 37.38 36.95 37.17 37.41 37.23 TiOZ _____________ .02 .03 .04 .01 .07 .04 .06 A1203 _____________ 21.09 21.19 20.90 21.25 21.71 21.32 21.18 FeO ______________ 35.98 35.17 34.31 37.70 37.39 33.21 32.42 MnO _____________ 2.18 2.53 3.46 1.55 1.93 5.02 6.17 Mg‘O _____________ 1.71 1.46 1.30 1.62 1.56 1.50 1.33 03.0 ______________ 2.67 3.43 3.33 2.59 2.65 3.57 3.42 Na20 _____________ .00 .01 .00 .01 .0‘0 .00 .02 K20 ______________ .01 .01 .01 .02 .01 .00 .01 F ________________ nd nd nd nd nd nd nd Sum _______ 100.39 101.11 100.73 101.70 102.49 102.07 101.84 Number of atoms Si ________________ 2.98 2.99 3.01 2.96 2.96 2.98 2.98 Ti ________________ .00 .00 .00 .00 .00 .00 .00 Al _______________ 2 01 2 00 1.99 2.01 2.03 2.00 2.00 Fe _______________ 2 44 2 36 2.31 2.53 2 49 2 21 2 17 Mn ______________ 15 17 .24 .11 13 34 42 Mg ______________ 21 17 .16 .19 19 18 16 Ca _______________ 23 29 .29 22 23 30 29 Na _______________ 00 00 .00 .00 00 00 00 K ________________ 00 00 .00 .00 00 00 00 F ________________ nd nd nd nd nd nd nd Cation sum ___ 8.02 7.98 8.00 8.02 8.03 8.01 8.02 Notes In contact Next to Rt. Next to Bt Rim of Part way Farther Center of with 182012; 182012; crystal into into crystal: Cd part farther contain- crystal crystal- near Cd 182008; way into ing contain- contain- 191010 next to into crystal spots ing ing and 11m Bt crystal contain- 191005 spots spots 191011 182012; contain- ing and 191004 191004 on rim ‘ ing spots 191007 and and of spots 182009 191007 191005 crystal 182009 and than contain- and 182010 spot ing 182011 than 191005 spots spot 182010 182010 and 182011 104 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 3.—Microprobe data on various minerals in samples of rocks from within and around the Taconic allochthon—Continued F. GARNET—Continued Sample ________________ 655—1 1052—2 Spot ___________________ 1 2 3 4 2 3 4 5 6 Weight percent Si02 _____________ 37.54 37.18 37.27 36.94 36.69 37.19 35.86 36.98 35.48 T102 _____________ .09 .14 .11 .03 .01 .02 .05 .15 .23 A1203 _____________ 21.34 20.57 21.10 20.76 21.13 20.85 21.95 21.21 21.56 FeO ______________ 34.30 35.18 33.72 36.34 37.17 37.12 32.54 30.34 26.00 MnO _____________ .73 .50 1.88 1.48 1.73 1.87 3.48 6.45 7.98 MgO _____________ 1.79 1.27 .98 2.07 1.99 2.46 1.28 .69 .84 CaO ______________ 5.29 5.38 5.46 2.27 1.17 1.37 4.86 5.09 6.30 Na20 _____________ nd nd nd nd nd nd .03 nd .04 K20 ______________ nd nd nd nd nd nd .00 nd .00 F ________________ nd nd nd nd nd nd nd nd nd Sum _______ 101.08 100.22 100.52 99.89 99.89 100.88 100.05 100.91 98.43 Number of atoms Si ________________ 2.99 3.01 3.00 3.00 2.98 2.99 2.91 2.98 2.92 Ti ________________ .00 .00 .00 .00 .00 .00 .00 .01 .01 A1 _______________ 2.00 1.96 2.00 1.99 2.02 1.98 2.10 2.01 2.09 Fe _______________ 2.28 2.38 2.27 2.47 2.53 2.50 2.21 2.04 1.79 Mn ______________ .05 .03 .13 .10 .12 .13 .24 .44 .55 Mg ______________ .21 .15 .12 .25 .24 .30 .15 .08 .10 Ca _______________ .45 .47 .47 .20 .10 .12 .42 .44 .55 Na _______________ nd nd nd nd nd nd .00 nd .00 K ________________ nd nd nd nd nd nd .00 nd .00 F ________________ nd nd nd nd nd nd nd nd nd Cation sum ___ 7.98 8.01 7.99 8.01 7.99 8.02 8.03 8.00 8.01 Notes ZnO:0.00 TABLES 105 TABLE 3,—Microprobe data on various minerals in samples of rocks from within and around the Tacom'c allochthon—Continued F. GARNET—Continued 1062—2—Continued 7 8 9 10 1 1 Weight percent 36.90 36.09 37.32 36.87 36.43 .24 .12 .23 .14 .00 21.18 20.75 20.62 20.87 21.14 25.11 26.09 29.74 34.34 37.70 10.55 9.45 6.24 2.10 1.47 .44 .49 .89 2.05 1.79 5.58 6.79 5.85 3.29 1.61 nd .03 nd .04 .05 nd 00 nd 00 00 nd nd nd nd .00 100.00 99.84 100.89 99.70 100.19 Number of atoms 2.99 2.95 3.00 2.99 2.96 .01 .01 .01 .01 .00 2.02 2.00 1.95 2.00 2.03 1.70 1.78 2.00 2.33 2.56 .72 .65 .42 .14 .10 .05 .06 .11 .25 .22 .48 .59 .50 .28 .14 nd .00 nd .00 .00 nd .00 nd .00 .00 nd nd nd nd .00 Cation sum ___ 7.97 8.04 7.99 8.00 8.01 Notes Weight Zn0=0.00 Zn0:0.00 percept sum 1n- eludes ZnO : 0.03 106 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 3.—Microprobe data on various minerals in samples of rocks from within and around the Taconic allochthon—Continued G. KYANITE Sample ________________ 655—1 Spot ___________________ 1 2 3 Weight percent S102 _____________ 37.21 37.60 37 69 T102 _____________ .03 .04 03 A1203 _____________ 60.43 60 86 61 23 FeO ______________ .15 64 16 MnO _____________ .00 03 00 MgO _____________ .00 00 00 CaO ______________ .07 04 05 Na20 _____________ nd nd nd 2 ______________ nd nd nd ________________ nd nd nd Sum _______ 98 07 99.51 99 16 Number of atoms S1 ________________ 1.02 1.02 1 02 T1 ________________ .00 .00 00 Al _______________ 1.96 1.95 1 96 Fe _______________ .00 .01 00 Mn ______________ .00 00 00 Mg ______________ .00 00 00 Ca. _______________ .00 00 00 Na _______________ nd nd nd K ________________ nd nd nd F ________________ nd nd nd Cation sum ___ 2.99 2.99 2.98 Notes Weight Weight percept percent sum m- sum m- cludes eludes ZnO ZnO : 0.18; = 0.30; catior} catim} sum ln- sum m- eludes eludes 0.01 0.01 atom atom Zn Zn TABLES 107 TABLE 3.——Mic'rop’robe data, on various minerals in samples of rocks from within and around the Taconic allochthon—Continued H. HORNBLENDE2 and CUMMINGTONITE Sample ________________ 1 6 1—-1 (hornblende) Spot ___________________ 1 2 3 4 5 Weight percent 39.39 39.35 38.55 40.45 39.66 .16 .05 .22 .07 .25 19.50 19.96 18.36 18.03 18.55 23.49 21.86 23.62 22.61 22.43 1.48 .00 .13 1.77 1.05 3.46 3.55 3.62 3.57 3.35 9.88 9.98 9.85 10.30 10.06 1.30 1.34 1.53 1.31 1.29 .05 .00 .00 .1 1 .00 .28 .1 3 .22 .29 .1 7 98.83 96.24 95.94 98.30 96.70 1.97 1.95 1.92 1.97 1.95 100.80 98.19 97.86 100.27 98.65 Number of atoms 5.98 6.04 6.02 6.16 6.11 .02 .00 .02 .0-0 .03 3.49 3.61 3.38 3.24 3.37 2.98 2.81 3.08 2.88 2.89 .1 9 .00 .01 .23 .14 .78 .81 .84 .81 .77 1.60 1.64 1.65 1.68 1.66 .38 .40 .46 .38 .38 .01 .00 .00 .02 .00 .13 .06 .11 .14 .08 Cation sum ___ 15.43 15.31 15.46 15.40 15.35 Notes BaO : 0; Weight BaO = 0; REG 2 0; B30 = 0; weight percent weight weight weight percent sum in- percent percent percent sum in- eludes sum in- sum in- sum in- clu des BaO eludes eludes eludes C1 = 0.09 Cl Cl =0.14; and Cl 20.04; =0.07; 20.06; Fluorine : 0.08; Fluorine Fluorine Fluorine cation Fluorine cation cation cation sum cation sum sum sum includes sum includes includes includes 0.03 includes 0.01 0.02 . atom Cl 0.02 atom Cl atom 01 atom 01 atom Cl See footnotes at end of table, 9. 123. 108 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 3.——Microprobe data on various minerals in samples of rocks from within and around the Tacom'c allochthon—Continued H. HORNBLENDE2 and CUMMINGTONITE—Continued Sample ________________ 487—2—4 (cummingtonite) Weight percent 47.82 48.30 .16 .00 .35 .98 36.69 36.82 .00 .55 8.13 8.39 .23 .13 .03 .05 nd nd .14 .02 93.43 95 22 1.82 1.86 95.25 97.08 Number of atoms 7.88 7.81 .02 .00 .06 .18 5.05 4.97 .00 .07 2.00 2.02 .04 .02 .00 .00 nd nd .07 .01 Cation sum ___ 15.05 15.07 See footnotes at end of table, p. 123. TABLES 109 TABLE 3.—Mic'rop'robe data on various minerals in samples of rocks from within and around the Tacom'c allochthon—Con-tinued I. EPIDOTE Spot __________________ 211001 213002 213003 Weight percent 37.15 37.06 36.34 .08 .24 .13 24.75 24.52 23.79 10.59 11.34 12.7 .13 .21 .20 05 .14 06 23 34 22.92 21 31 02 .03 04 06 .01 00 nd nd nd 96.17 96.47 94 58 3.65 3.65 3 57 99.82 100.12 98 15 Number of atoms 2.93 2.93 2 94 .00 .01 01 2 30 2.28 2 27 70 .75 86 01 .01 0‘1 01 .02 01 1 97 1.94 1 86 00 .00 01 00 .00 00 nd nd nd Cation sum ___ 7.92 7.94 7.97 Notes Iron com— Iron co'm- Iron com- puted as puted as puted as ferric; ferric; ferric next to next to Mu Mu 211 002 21 3005 110 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 3.—Microprobe data on various minerals in samples of rocks from within and around the Tacom'c allochthon~Continued J. PLAGIOCLASE 3—3 14—1 103—1 23 3 4 4a. 4 5 21 052009 052010 Weight percent SiOa ______________ 66.89 66.58 66.83 64.18 62.64 62.23 61.14 64.74 66.80 T102 _____________ n1 nd nd nd nd nd nd .03 .00 A1203 _____________ 210.58 20.59 19.80 21.39 24.41 25.01 24.06 21.83 20.39 FeO ______________ nd nd nd nd nd nd nd .13 .16 MnO _____________ nd nd nd nd nd nd nd 00 02 MgO _____________ mi nd nd nd nd nd nd 03 01 03.0 ______________ 95 67 48 2.22 5 14 5 17 5 20 2 46 Na20 _____________ 11 14 11 33 11 34 10.45 8.68 8 78 8 66 9 95 11 38 2 ______________ 00 01 00 .00 .02 02 03 09 ________________ nd nd nd nd nd nd nd nd nd Sum _______ 99.56 99.18 98.45 98.24 100.89 101.21 99.09 99.26 99.61 Number of atoms Si ________________ 2.941 2 939 2 968 2 874 2.748 2.725 2 736 2 868 2 941 Ti ________________ n-. n nd nd nd nd 11 000 000 Al _______________ 1.066 1 071 1 036 1 129 1.262 1.290 1 269 1 139 1 058 Fe _______________ nd nd nd nd nd nd nd 005 006 Mn _______________ nd nd nd nd nd nd nd 000 000 Mg _______________ nd nd nd nd nd nd nd 002 000 Ca _______________ 045 032 023 .106 .242 242 249 117 036 Na _______________ 949 969 976 .9-07 .738 745 751 854 971 K ________________ 000 001 000 .000 .001 001 002 005 005 F ________________ nd nd nd nd nd nd nd nd nd Cation sum ___ 5 001 5.012 5.003 5.016 4.991 5.003 5.007 4.990 5.017 Notes 4.007 4.010 4.004 4.003 4.010 4.015 4.005 4.007 3.999 .994 1.002 .999 1.013 .981 .988 1.002 .983 1.018 —.027 —.038 —.014 —.010 —.o39 —.059 —.018 —.032 +.002 See footnotes at end of table, 1). 123. TABLES 1 1 1 TABLE 3.—Mic’roprobe data on various minerals in samples of rocks from within and around the Taconic allochthon—Continued J. PLAGIOCLASE—Continued Sample ................ 103—2 161—1 Spot __________________ 061015 061016 061018 062013 062014 1 2 Weight percent SiOa ______________ 59.56 59.82 61.25 59.53 59.52 58.84 58.65 T102 _____________ .12 .03 00 .02 .01 nd nd A1203 _____________ 24 70 24 53 23.77 24 80 25.40 25 52 25 64 FeO ______________ 37 17 .19 .17 10 nd nd MnO _____________ 00 06 .01 .01 00 nd nd MgO _____________ 02 01 .03 .03 00 nd nd CaO ______________ 6 30 5 70 5.17 6.60 6 57 6 74 6 65 Na20 _____________ 7 73 8 75 9.15 8.14 8 40 7 81 813 2 ______________ 06 05 .06 .08 07 18 11 ________________ nd nd nd nd nd nd nd Sum _______ 98.86 99.12 99.63 99.38 100.07 99.09 99.18 Number of atoms Si ________________ 2.683 2.691 2.735 2.674 2.656 2.649 2.641 Ti ________________ .004 .004 .000 .001 .000 nd nd Al _______________ 1.311 1.300 1.251 1.312 1.336 1.354 1.360 Fe _______________ .014 .006 .007 .006 .000 nd nd Mn _______________ .000 .002 .000 .000 .000 nd nd Mg _______________ .001 .001 .000 .000 .000 nd nd Ca _______________ .304 .275 .247 .317 .314 .325 .321 Na _______________ .675 .763 .792 .708 .726 .681 .709 K ________________ .003 .003 .003 .005 .004 .010 .006 F ________________ nd nd nd nd nd nd nd Cation sum __.. 4.995 5.045 5.035 5.023 5.036 5.019 5.037 Notes Next to Near core Next to Spots 062013 and 062014 Mn 0 Mu are in the same crystal 961014 crysth 061017 111 contam- crystal ing contain- spot ing 061015 spot 061016 Z 3 ____________________ 3.998 3.995 3.986 3.987 3.992 4.003 4.001 X ..................... .997 1.050 1.049 1.036 1.044 1.016 < 1.036 Y _____________________ +.005 +.033 +.052 +.047 +.022 —.013 +.007 See footnotes at end of table, 1). 123. 112 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 3,—Microprobe data on various minerals in samples of rocks from within and around the Taconic allochthon—Continued ]. PLAGIOCLASE—Continued Sample ________________ 234-1 289—2 Spot __________________ 2 3 3A 17 17A 18 4 6 8 Weight percent Si02 ______________ 56.77 58.86 60.07 58.43 60.49 60.83 44.20 44.86 46.40 Ti02 _____________ nd nd nd nd nd nd nd nd 11 A1203 _____________ 26 82 26 15 24 66 25.58 25.02 24 95 34 77 33 92 35 26 ______________ 09 09 nd nd nd nd nd nd nd MnO _____________ nd nd nd nd nd nd nd nd nd MgO _____________ nd nd nd nd nd nd nd nd nd CaO ______________ 7 52 7 45 6 33 7.05 6.74 5 63 18 58 18 33 17 33 Na20 _____________ 7 11 7 26 8 08 7.54 8.00 8 16 86 82 1 66 a ______________ 04 05 00 .02 .03 03 17 16 17 F ________________ nd nd nd nd nd nd nd nd nd Sum _______ 98.35 99.86 99.14 98.62 100.28 99.60 98.58 98.09 100.82 Number of atoms Si _______________ 2.581 2.630 2.695 2.642 2.686 2.708 2.072 2.109 2.116 Ti _______________ nd nd nd nd nd nd nd nd nd Al _______________ 1.437 1.377 1.304 1.363 1.309 1.309 1.920 1.879 1.895 Fe _______________ .003 .003 nd nd nd nd nd nd nd Mn ______________ nd nd nd nd nd nd nd nd nd Mg ______________ nd nd nd nd nd nd nd nd nd Ca _______________ 366 356 304 .341 .320 268 933 923 847 Na _______________ 626 629 702 .661 .688 704 078 075 147 K ________________ 002 003 000 001 .002 002 010 010 001 F ________________ nd nd nd nd nd nd nd nd nd Cation sum ___ 5.015 4.998 5.005 5.008 5.005 4.991 5.013 4.996 5.006 Notes za ____________________ 4.018 4.007 3.999 4.005 3.995 4.017 3.992 8.988 4.011 X _____________________ .997 .991 1.006 1.003 1.010 .974 1.021 1.008 .995 Y _____________________ —.071 —.027 +.006 —.018 +.021 —.067 +.034 +.062 —.054 See footnotes at end of table, p. 123. TABLES 113 TABLE 3,—Microp’robe data on various minerals in samples of rocks from within and around the Taconic allochthon—Continued J. PLAGIOCLASE—Continued Sample ________________ 338-1 339-1 356—1 360—1 Spot .................. 101204 112002 112003 112006 3 2 3 Weight percent 66.94 61.36 61.47 60.77 64.88 64.35 63.74 .02 .03 .02 .02 nd nd nd 21 77 25 08 25 00 24.93 21 24 22 19 22 77 16 12 17 .12 12 nd nd 03 01 02 .01 nd nd nd 01 03 02 .03 00 nd nd 1 54 5 86 5 83 5.97 3 01 2 52 3 28 11 13 8 83 8 78 8.59 10 25 10 15 9 75 12 06 07 .06 0 16 23 00 nd nd nd nd nd nd nd 101.72 101.38 101.38 100.50 99.81 99.44 99.54 Number of atoms Si _______________ 2.894 2.695 2.700 2.693 2.873 2.850 2.822 Ti _______________ .001 .001 .001 .000 nd nd nd Al _______________ 1.109 1.298 1.294 1.302 1.108 1.158 1.188 Fe _______________ .006 .004 .006 .004 .004 nd nd Mn ______________ .001 .000 .001 .000 nd nd nd Mg ______________ .001 .002 .001 .002 .000 nd nd Ca _______________ .071 .276 .274 .283 .143 .120 .156 Na _______________ .932 .752 .747 .738 .880 .871 .836 K ________________ .007 .003 .004 .003 .009 .013 .000 F ________________ nd nd nd nd nd nd nd Cation sum ___ 5.022 5.031 5.028 5.025 5.017 5.012 5.002 Notes Rim of Next to Core of Weight crystal Ch crystal percent contain- 112004; contain- sum ing in ing includes spots crystal spots BaO: 112003 contain- 112002 0.15 and ing and 112006 spots 112003 112002 and 112006 4.004 3.994 3.994 3.995 3.981 4.008 4.010 1.018 1.037 1.034 1.030 1.036 1.004 .992 —.014 +.022 +.020 +.017 +.075 —.035 ~.040 Sec footnotes at end of table, p. 123. 114 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 3.—Microprobe data on various minerals in samples of rocks from within and around the Taconic allochthon—Continued J. PLAGIOCLASE—Continued Sample ________________ 360—1—Continued 369—1 463—1 Spot __________________ 121001 121003 121004 122002 122003 131013 131014 141009 141012 Weight percent Si02 ______________ 67.23 64.15 63.63 62.81 67.89 62.33 61.62 64.25 62.72 TiOa _____________ .04 .05 .04 .06 .04 .07 .03 .01 .03 A1203 _____________ 20.67 22.80 23.36 23.73 20.50 23.87 23.96 22.52 23.70 FeO ______________ .17 .20 .10 .16 .17 .32 .23 .31 .16 MnO _____________ .02 .04 .03 .03 .03 .03 .00 .01 .01 MgO _____________ .02 .05 .03 .05 .05 .00 .03 .01 .02 CaO ______________ .85 3.85 3.85 4.39 .58 5.40 5.06 2.95 4.30 Na20 _____________ 10.73 10.09 9.77 9.66 11.84 8.80 8.84 10.09 9.38 K20 ______________ .05 .05 .06 .05 .05 .04 .04 .07 .06 F ________________ nd nd nd nd nd nd nd nd nd Sum _______ 99.78 101.28 100.87 100.94 101.15 100.86 99.81 100.22 100.38 Number of atoms Si _______________ 2.946 2.814 2.791 2.761 2.945 2.746 2.740 2.831 2.768 Ti _______________ .001 .000 .001 .002 .001 .002 .001 .000 .001 Al _______________ 1.067 1.180 1.207 1.229 1.048 1.239 1.256 1.169 1.232 Fe _______________ .006 .007 .004 .006 .006 .012 .009 .011 .006 Mn ______________ .001 .001 .001 .001 .001 .001 .000 .000 .000 Mg ______________ .001 .00-3 .002 .003 .003 .000 .002 .001 .001 Ca _______________ .040 .159 .181 .207 .027 .255 .241 .139 .203 Na _______________ .911 .858 .830 .823 .995 .751 .762 .861 .802 K ________________ .003 .003 .003 .003 .003 .002 .002 .004 .003 F ________________ nd nd nd nd nd nd nd nd nd Cation sum ___ 4.976 5.025 5.020 5.035 5.029 5.008 5.013 5.016 5.016 Notes On rim In crystal Just out- Core of Rim of Rim of In crystal Next to Next to of contain- side crystal crystal crystal contain- Cd Mu crystal ing core contain- contain- contain- ing 141008 141010 contain- spots zone of ing in: ing spot ing 121001 crystal spot spot spot 131013 spots and contain- 122003 122002 131014 121003 121004; ing and next to spots 121004: spot 121001 next to 121001 and spot and 121003 121003 11m 121007 Z3 _____________________ 4.014 3.994 3.999 3.992 3.994 3.987 3.997 4.000 4.001 X _____________________ .962 1.031 1.021 1.043 1.035 1.021 1.016 1.016 1.015 Y _____________________ —.057 +.025 +.002 +.030 +.025 +.050 +.012 —.001 —.005 Sec footnotes at end of table, 1). 123. TABLES 115 TABLE 3.—Microprobe data on various minerals in samples of rocks from within and around the Tacom'c allochthon—Con‘tinued J. PLAGIOCLASE—Continucd Sample ________________ 463—1—Continued 466—1 506—1 515—1 655—1 Spot __________________ 141021 142008 152003 174001 174002 191002 3 Weight percent 64.97 63.03 66.54 61.42 62.20 61.61 60.03 .16 .02 .01 .14 .07 .08 nd 22.23. 23.32 21.16 23.33 23.41 24.03 25.02 .27 .24 .15 .19 .15 .21 nd .01 .01 .01 .00 .00 .07 nd .02 .03 .01 .01 .01 .01 nd 3.12 4.16 1.11 5.02 4.82 5.04 5.73 10.21 9.66 11.77 9.36 9.47 9.11 8.16 .08 .09 .08 .07 .03 .07 .12 nd nd nd nd nd nd nd 101.07 100.56 100.84 99.54 100.16 100.23 99.06 Number of atoms Si _______________ 2.840 2.779 2.904 2.746 2.759 2.732 2.692 Ti _______________ .005 .001 .000 .005 .002 .003 nd Al _______________ 1.145 1.212 1.008 1.229 1.223 1.257 1.322 Fe _______________ .010 .009 .005 .007 .006 .008 nd Mn ______________ .000 .000 .000 .000 .000 .003 nd Mg ______________ .001 .002 .001 .001 .001 .001 nd Ca _______________ .146 .196 .052 .240 .229 .239 .275 Na _______________ .865 .825 .995 .811 .814 .783 .709 K ________________ .004 .0015 .004 .004 .002 .004 .007 F ________________ nd nd nd nd nd nd nd Cation sum ___ 5.016 5.029 5.049 5.043 5.036 5.030 5.005 Notes Next to In rim of In core In rim of Core of Next to Next to 11m crystal next to crystal crystal 11m kyanibe 141020 11 contain- contain- 191001 152004 in: ing 3.992 spot spot 1.038 174002 174001 +.030 Z3 _____________________ 3.990 3.991 3.992 3.980 3.984 4.014 X _____________________ 1.026 1.038 1.057 1.063 1.052 .991 Y _____________________ +.039 +.034 +.028 +.082 +.062 —.056 Sec footnotes at end of table, p. 123. 116 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 3.—Microp'robe data on various minerals in samples of rocks from within and around the Taconic allochthon—Continued J. PLAGIOCLASE—Continued Sample ________________ 1052—2 1169—1 Spot __________________ 13 213008 213011 Weight percent 8102 ______________ 65.84 68.19 68 09 T10, _____________ nd .07 00 A1203 _____________ 21.07 19.83 19 74 FeO ______________ nd .22 14 MnO _____________ nd .02 03 MgO _____________ nd .00 05 CaO ______________ 1.32 .02 04 Na20 _____________ 11.46 11.91 12 10 2 ______________ .17 .03 03 F ________________ nd nd nd Sum _______ 99 86 100.29 100 22 Number of atoms SI _______________ 2 901 2.976 2 975 T1 _______________ nd .002 000 A1 _______________ 1 094 1.019 1 016 Fe _______________ nd .008 005 Mn ______________ nd .001 001 Mg ______________ nd .000 003 Ca _______________ 062 .001 002 Na _______________ .979 1.007 1.025 K ________________ .010 .0010 .002 F ________________ nd nd nd Cation sum ___ 5.046 5.014 5.029 Notes Next to Ilm 213007 3.995 3.997 3.991 1.051 1.017 1.038 +.019 +.009 +-033 See footnotes at end of table, p. 123. TABLES 117 TABLE 3.—Microprobe data on various minerals in samples of rocks from within and around the Taconic allochthon—Continued K. ILMENITE Sample ________________ 14—1 102-1 103—1 103-2 290—1 Spot .................. 6 1 2 3 052003 061013 062005 7 8 Weight percent Si02 ______________ nd 0.85 0.39 0.15 0.23 0.11 0.19 0.22 0.30 Ti02 _____________ 53.94 52.38 52.17 52.46 52.22 52.24 53.22 52.94 52.59 A1203 _____________ nd .06 .10 .13 .51 .37 .45 .12 .12 FeO ______________ 45.81 45.99 46.77 45.72 46.65 47.07 46.21 46.89 46.76 MnO _____________ nd .62 .70 .55 .58 .63 .91 .00 .02 MgO _____________ .02 .10 .13 .14 .11 .34 .07 .11 .17 CaO _____________ nd .02 .06 .26 .08 .06 .05 .06 .04 Na40 _____________ nd nd nd nd .00 .00 .00 nd nd K20 ______________ nd nd nd nd .10 .01 .06 n:d nd F ________________ nd nd nd nd nd nd nd nd nd Sum _______ 99.77 100.09 100.35 99.41 100.48 100.83 101.16 100.28 100.00 Number of atoms n'rd 0.02 0.01 0.00 0.01 0.00 0.00 0.01 0.01 1.02 .99 .99 1.00 .98 .98 .99 1.00 .99 nd .00 .00 .00 .01 .01 .01 .00 .00 .96 .96 .98 .97 .98 .98 .96 .98 .98 nd .01 .01 01 .01 .01 .02 .00 .00 .00 .00 .0-0 01 .00 .01 .00 .00 .01 nd .00 .00 01 .00 .0-0 .00 .00 .00 nd nd nd nd .00 .00 .00 nd nd nd nd nd nd .00 .00 .00 nd nd nd n-d nd nd nd nd nd nd nd Cation sum ___ 1.98 1.98 1.99 2.00 1.99 1.99 1.98 1.99 1.99 Notes Zn0=0.00 Weight Weight Cr203=0 Near Mu percent percent 052002 sum in- sum in- and Gil eludes eludes 052001 01‘an Cr203 20.07 20.03 118 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 3.—Microp’robe data on various minerals in samples of rocks from within and around the Tacoma allochthon—Continued K. ILMENITE—Continued Sample ________________ 290—1—Con. 331—1 338-1 339—1 Spot __________________ 10 091002 092001 093008 102001 102002 1 2 Weight percent $102 ______________ 0.23 0.18 0.17 0.10 0.11 0.15 0.05 0.13 T102 _____________ 52.76 54.72 54.34 54.40 52.91 53.13 53.32 52.81 A1203 _____________ .13 .45 .47 .53 .49 .46 .16 .23 FeO ______________ 47.53 45.69 45.87 44.51 48.07 48.30 47.05 46.24 MnO _____________ .07 .48 .54 .54 .41 .38 .09 .16 Mg‘O _____________ .13 .18 .16 .13 .07 .20 .18 .06 CaO _____________ .07 .00 .00 .02 .02 .0‘1 .02 .03 Naeo _____________ nd .02 .0’1 .00 .03 .00 nd nd K20 ______________ nd .02 .03 .02 .04 .05 nd nd F ________________ nd nd nd nd nd nd nd nd Sum _______ 100.92 101.74 101.59 100.25 102.15 102.68 100.93 99.66 Number of atoms Si ________________ 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ' .99 1.01 1.01 1.02 .98 .98 1.00 1.00 .00 .01 .0’1 .02 .01 .01 .00 .01 .99 .94 .94 .92 .99 .99 .98 .97 .00 .01 .01 .01 .01 .01 .00 .00 .00 .01 .01 .00 .00 .00 .01 .00 .00 .00 .00 .00 .00 .00 .00 .06 nd .00 .00 .00 .00 .00 n-d nd nd .00 .00 .00 .00 .00 nd nd nd nd nd nd nd nd nd nd Cation sum -__ 1.99 1.98 1.98 1.97 1.99 1.99 1.99 1.98 Notes Next to Next to Next to Next to Weight Cr203 Cd Mu Cd 11 percent = 0.00 091001 092002 102005 102004 sum in- eludes Cr203 : 0.06 TABLES 119 TABLE 3.—Microprobe data on various minerals in samples of rocks from within and around the Taconic allochthon—Continued K. ILMENITE—Continued Sample ________________ 339—1—Continued 355—1 Spot __________________ 3 4 5 6 7 7 8 Weight percent Si02 ______________ 0.14 0.22 0.18 0.23 0.14 mi Ti02 _____________ 52 77 53 12 53.62 53.61 52 13 51 76 50 91 A1203 _____________ 25 16 .12 .39 13 nd nd FeO ______________ 45 84 44 71 45.19 44.70 45 53 47 40 48 63 MnO _____________ 14 32 .35 12 13 nd nd MgO _____________ 05 36 .30 22 37 33 37 030 _____________ 00 03 .00 00 02 nd nd Na/gO _____________ nd nd nd nd nd nd nd 2 ______________ nd nd nd nd nd nd nd F ________________ nd nd nd nd nd nd nd Sum _______ 99.19 98 95 99.77 99.32 98 45 99 49 9‘9 91 Number of atoms 0.00 0 01 0 00 0.01 0 00 nd nd 1 00 1 01 1 01 1.01 1 00 0 99 0 98 01 00 00 .01 00 nd nd 97 94 95 .94 97 1 01 1 04 00 01 01 .00 00 nd nd 00 01 01 .01 01 01 01 0‘0 00 00 .00 00 nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd 1131 nd nd nd nd nd Cation sum ___ 1.98 1.98 1.98 1.98 1.98 2.01 2.03 Notes Cr203 Weight Weight Weight Cr203 ZnO : 0.00 ZnO = 0.00 : 0.00 percept percept percept = 0.00 sum ln- sum ln- sum 1n- cludes eludes eludes CraOs Cran CrzOs 20.03 = 0.01 = 0.05 120 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 3.—Microprobe data on various minerals in samples of rocks from within and around the Taconic allochthon—Con-tinued K. ILMENITE—Continued Sample ________________ 35 5—1—Continued 356—1 Spot .................. 16a 16 16 17 18 25 36 37 38 Weight percent Si02 ______________ nd nd 0.21 0.33 0.47 0.24 0.48 0.21 0.22 T102 _____________ 52.58 51.78 52.87 49.74 51.62 53.93 53.97 49.53 52.13 A1203 _____________ nd nd .20 .36 .23 .25 .38 .19 .23 FeO ______________ 46 68 46 29 46.38 47.13 47 77 46 51 47 20 48 74 47 84 MnO _____________ 11 nd .74 .85 2 16 1 14 19 MgO _____________ 00 00 .22 .24 46 21 14 42 47 03.0 _____________ nd nd 02 .18 07 02 00 00 00 Naeo _____________ nd nd 04 .00 00 00 06 04 02 K20 ______________ nd nd 00 .00 00 00 00 00 00 ________________ nd nd nd nd nd nd nd nd nd Sum _______ 99.26 98.07 100.78 98.83 100.82 101.53 102.43 99.64 101.30 Number of atoms Sl ________________ nd nd 0 01 0.01 0.01 0.01 0 01 0 01 0 01 T1 _______________ 1 00 1 00 99 .96 .97 1 00 99 95 98 A1 _______________ nd nd 01 .01 .0-1 01 01 01 01 Fe _______________ 99 1 00 97 1.01 1.00 96 96 1 04 1 00 Mn ______________ nd nd 02 .02 .00 00 00 00 Mg ______________ 00 00 01 .01 .02 01 01 02 02 Ca _______________ nd nd 00 .00 .00 00 00 00 00 Na _______________ nd nd 00 .00 .00 00 00 00 00 K ________________ nd nd 00 .00 .00 00 00 00 00 F ________________ nd nd nd nd nd nd nd nd nd Cation sum ___ 1.99 2.00 2.01 2.02 2.01 1.99 1.98 2.04 2.02 Notes ZnO ZnO In Ga con- ZnO ZnO In matrix; In matrix; In Ga con- In Ga con- : . =0.00 mining 20.00; 20.00; Weight weight taining taining spot in Ga in Ga percent percent spot spot Ga 9; contain- contain- sum in- sum in- Ga 45; Ga 40; weight ing ing eludes eludes weight weight percent spot spot ZnO ZnO percent percent sum in- Ga 8 Ga 3 20.21 20.07 sum in- sum in- cludes eludes eludes ZnO ZnO ZnO 20.10 20.37; 20.20 cation sum m- cluds 0.01 atom Zn TABLES 121 TABLE 3.~—Microprobe data on various minerals in samples of rocks from within and around the Taconic allochthon—Continued K. ILMENITE—~Continued Sample ________________ ‘360—1 269—1 463—1 Spot __________________ 101006 121007 122001 1 2 3 4 Weight percent 8102 ______________ 0.32 0.22 1.85 0.26 0.77 0.16 0.21 TiO, _____________ 49.517 51.82 50.29 51.58 51.41 52.12 51.25 A1203 _____________ .45 .49 .49 .09 .20 .20 .31 FeO ______________ 46.16 45.46 46.28 46.95 46.25 45.85 47.27 MnO _____________ 1.90 2.16 1.99 23 .20 .71 .63 MgO _____________ .13 .12 .09 11 .11 .01 .03 03.0 _____________ .12 .10 .06 05 .14 .00 .02 NaqO _____________ .04 .02 .02 nd nd nd nd K20 ______________ .03 .043 .04 nd nd nd nd ________________ nd nd nd nd nd nd nd Sum _______ 98.72 100.42 101.11 99.27 99.08 99.05 99.72 Number of atoms Si ________________ 0.101 0.01 0 05 0.01 0.02 01.00 0.01 Ti _______________ .96 .98 94 .99 .98 1.00 .98 Al _______________ .01 .01 01 .00 .01 .‘0'1 .01 Fe _______________ .99 .96 .96 1.00 .98 .97 1.00 Mn ______________ .04 .05 .04 .00 .00 .02 .01 Mg ______________ .00 .0'0 .0‘0‘ .00 .00 .00 .00 Ca _______________ .00 .00 .00 .00 .00 .00 .00 Na _______________ .00 .00 .00 nd nd nd nd K ________________ .00 .00 .00 nd nd nd nd F ________________ nd nd nd nd nd nd nd Cation sum ___ 2.01 2.01 2.00 2.00 1.99 2.00 2.01 Notes Next to P1; 121003 122 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 3.—Microp'robe data on various minerals in samples of rocks from within and around the Tacom'c allochthon—Continuevd K. ILMENITE—Continued Sample ________________ 515-1 1169—1 Spot __________________ 191001 191009 191011 191027 1 2 3 215001 3 Weight percent 8102 ______________ 0.15 0.17 0.14 0.17 0.13 0.19 0.24 0.25 0.33 T102 _____________ 52.34 53.06 53.63 53.06 51.61 51.50 51.26 52.29 50.76 A1203 _____________ .42 .39 .43 .59 .23 .14 .18 .48 .07 FeO ______________ 46.94 47.44 46.09 46.72 47.37 46.84 46.26 45.47 44.75 MnO _____________ .35 .35 1.16 .48 .25 .26 .35 2.97 2.79 MgO _____________ .21 .22 .20 .12 .019 .13 .11 .07 .00 C30 _____________ .03 .0'8 .10 .02 .00 .03 .02 .03 .02 NaaO _____________ .00 .00 .05 .05 nd nd nd .04 nd K20 ______________ .04 .02 .02 .08 nd nd nd .04 nd F ________________ nd nd nd nd nd nd nd nd nd Sum _______ 100.48 101.73 101.82 101.29 99.68 99.09 98.42 101.64 98.72 Number of atoms Si ________________ 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.01 Ti _______________ .99 99 .99 .99 .98 .99 .99 .98 98 Al _______________ 01 01 01 .02 .01 00 01 01 00 Fe _______________ 98 98 95 .97 1.01 1 00 99 94 96 Mn ______________ 01 01 02 .01 .01 01 01 06 06 Mg ______________ 01 01 01 .00 .00 00 00 00 00 Ca. _______________ 00 00 00 .00 .00 00 00 00 00 Na _______________ 00 00 00 .00 nd nd nd 00 nd K ________________ 00 00 00 .00 n-d nd nd 00 nd F ________________ nd nd nd nd nd nd nd nd nd Cation sum ___ 2.00 2.00 1.98 1.99 2.01 2.00 2.01 2.00 2.01 Notes In Pg con— In Ga con- In Ga con- Next to taining taining taining Mu spot Pg spot Ga spot Ga 215003 191002: 191004; 191004; next to next to near Ga Mu d 191012 191003 191008 TABLES 123 TABLE 3.—Microprobe data, on various minerals in samples of rocks from within and around the Tacom'c allochthon—Continued L. MAGNETITE Sample ................ 191-1 Spot __________________ Weight percent Number of atoms Cation sum _:_ 3.66 Notes ZnO =0.00; sum is 97.51 when calcu- lated as Fe304; cation sum based on 4 oxygen per formula and % FeOTotn] as Fean 1The “calculated weight percent H20” of the staurolite analyses is based on an anhydrous formula of 23.5 oxygen atoms per formula of 24 oxygens. To obtain the calculated weight percent H20 on the basis of an ‘anhydrous formula of 23 oxygen atoms, multiply the H20 value by 2.04. ”Microprobe data for hornblende from samples 102—1 and 289—2 are given in Doolan and others (1978). 3 For definition of Z, X. and Y, see p. 20. 124 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 4.——Comrentional wet-chemical analyses and the number of atoms calculated on the basis of the anhydrous formulas for selected minerals [nd, not determined] Mineral _____________ Staurolite (313111111119 Muscovite 1 Sam Is no. ___________ p 355—1 5 356—1 355‘1 5 ‘356—1 355—1 356-1 140—2 Weight percent 28.55 29.45 37.46 37.95 45.5 46.7 47.3 .41 .58 .10 .09 .23 .23 .18 53.54 52.88 20.64 21.87 36.4 37.0 35.3 2.54 1.1 .48 2.3 2.0 .41 1.2 11.42 12.8 33.91 32.6 .40 .35 .56 .08 nd 3.28 .92 .03 .04 .03 1.06 1.55 1.75 2.29 .44 .41 .5 .09 .09 2.11 2.55 .15 .0 .0 nd nd .05 nd 2.3 2.0 1.4 <.05 nd (.01 ml 7.9 8.5 8.4 1.61 1.19 (.05 ml 4.5 )4 7 4.4 .02 <.02 .03 nd .08 ' .04 <.02 nd (.01 nd <.05 nd nd .03 nd .04 nd .00 05 ’ .04 .26 .23 nd nd nd nd nd 6 99.61 l’999 699.815 100.57 100.0 100.4 99.4 Number of atoms (7) (7) (3) (8) (9) (9) 0’) (1°) 4.02 4.13 3.04 3.01 3.01 3.06 3.12 .04 .06 .01 .01 .01 .01 .01 8.89 8.73 1.97 2.04 2.84 2.86 2.75 .27 .12 .03 .14 .10 .02 .06 1.35 1.50 2.30 2.16 .02 .02 .03 .01 nd .23 .05 00 .00 00 22 32 21 .27 04 04 05 01 01 18 .22 01 00 00 nd nd .00 nd 29 25 .18 <.01 nd .00 nd 67 71 .71 .03 02 .00 nd nd nd nd 1.5 1 1 ___________________________________ Optical data [Refractive index; estimated uncertainties in parentheses; sodium light] 1.742 1.739 1.815 1.807 1.564 1.568 at] ($002) (42002) ($.005) (i.005) (i.002) ($.005) 1.747 1.745 1.597 1.591 1.595 (i.002) (+.001) (i.002) (+.002) (i002) 1.754 1.754 1.60 1.598 1.598 ($.001) (+.002) (*1001) (+.002) (1.002) +~80° +~80° nd nd nd +811° +79” —45° — 57° nd (r>V) Cell data [Cell edges in angstroms; standard error in parentheses; interaxial angles in degrees, minutes, and fractions of minutes; cell volumes in cubic ang- stroms; molar volumes in cubic centimeters] a ____________________ 7.864 7.866 11.649 11.549 5.169 5.183 5.180 (1.001) ($002) (i001) (i901) (4:013) (i904) (+003) b ____________________ 16.617 16.631 9.002 8.979 9.013 (+.002) (i905) (+.020) (i.006) (+.005) c ____________________ 5.65 5.658 19.923 19.964 20.001 (:.001) (i002) (IL-.022) (1.005) (1006) a ____________________ 90" ° 90° 90° 90° B ____________________ 90° 90° 95°34.0’ 95°45.6’ 95°43.0’ ______________ ($100) (11.7’) (1:2.7’) 'y ____________________ 90° 90° 90° 90° 90° Cell volume __________ 739.5 740.2 922.7 924.3 929.1 (:2) (:3) ($2.2) (:6) (1.5) Molar volume ________ 222.7 222.9 138.9 139.2 139.9 (:J) (:J) (1.3) (i.1) (i1) Specific gravity calculated _________ 3.78 3.77 2.85 2.83 2.82 1 Muscovite is indexed on the basis of 2M1 polyty‘pe. 2 Biotite is indexed on the basis of 1M polytype. 3 Chloritoid is indexed on triclinic cell. 4 Chlorite is the IIb polytype. 5Special precision analyses by Ellen Lillie under supervision of Robert Meyrowitz, both of the U.S. Geological Survey. Approximately 6 mg of a sample was decomposed by fusion with NaOH in a silver crucible at 800° C; the melt was dissolved in dilute HCl; aliquots of this solution were used for the determination of SiOz and A1203. Si02 was determined spectrophoto- metrically by the molybdenum blue procedure using 1-amino—2-naphthol-4- sulfonic-acid-sulfite as the reducing agent. A1203 was determined spectro- photometrically by the alizari‘n red S-calcium procedure; the standard solu- tions contained approximately the same concentration of total iron present in the sample solution. Approximately 50 mg of sample was decomposed by fusion at 900° C with NazCOs; the‘ melt was leached with water and treated with HF and H0104 to remove silica; aliquots of this solution were used for the determination of total iron, MgO, CaO. K20, Ti02, P205, MnO, and ZnO. Total iron was determined spectrophotometrically with o-phenan- throline; the Fean equivalent of FeO prsent in the sample was sub- tracted from the total iron value to give Fe203. MgO was determined by atomic-absorption spectrophotometry; lanthanum was used as a releasing agent; the standard solutions contained approximately the same concentra- tion of H010; and sodium present in the sample solution. 080 was de- termined by atomic-absorption spectrophotometry; lanthanum was used as a releasing agent; the standard solutions contained approximately the same concentration of H0101, sodium, and MgO present in the sample solution. K20 and ZnO were determined by atomic-absorption spectrophotometry; the standard solutions contained approximately the same concentration of H0104 and sodium praent in the sample solution. TiOz was' determined spectro- photometrically by the “Tiron” procedure. P205 was determined spectro- photometrically by the heteropoly blue procedure. MnO was determined by TABLES 125 TABLE 4.——Conventional wet-chemical analyses and the number of atoms calculated on the basis of the anhydrous formulas for selected minerals—Continued Biotite 2 Chloritoid 3 Chlorite 4 Ilmenite Tremohte 356—1 140—2 355-1 140—2 355—1 140—2 918—1 Weight percent 36.8 26.8 25.8 24.4 1.2 3.2 58.25 1.6 1.0 .21 .15 46.8 48.7 .04 20.4 38.2 24.2 23.3 2.0 4.4 .53 1.6 5.3 2.8 3.2 11.6 2.2 .08 16.6 18.3 24.7 29.1 36.6 38.6 .24 .02 1.3 .05 .40 .05 1.2 .01 9.3 1.8 10.3 8.0 .23 .28 24.46 .00 .00 .00 1.0 .15 .00 13.47 .44 .09 .22 .08 .00 .28 .20‘ 8.3 .12 .56 .20 .14 .20 .04 } 4 3 6.7 10.0 10.0 .37 .30 2.03 ‘ .10 .70 .07 .02 .0‘0 .01 nd nd nd nd <.05 nd nd .12 .17 .15 .13 .02 .19 .00 nd nd nd nd nd nd mi 99.5 99.9 99.7 100.0 99.2 99.6 99.48 Number of atoms (a) (u) (12) (‘3) (13) (1‘) (15) (‘5) O“) 2.75 1.09 2.69 2.59 0.03 0.08 7.930 .09 .03 .02 .01 .87 .89 .000 1.80 1.83 2.97 2.92 .06 .13 .085 .09 .16 .22 .26 .22 .04 .008 1.04 .62 2.15 2.59 .76 .78 .027 .00 .04 .00 .04 .00 .02 .001 1.03 .11 1.60 1.27 .01 .01 4.960 .00 .00 .00 .11 .00 .00 1.964 .06 .01 .04 .02 .00 .01 .055 .79 .01 .07 .03 .00 .01 .007 nd nd nd nd nd nd nd __________________________________________ 1.933 Optical data [Refractive index; estimated uncertainties in parentheses; sodum light] a. ____________________ 1.588 1.722 nd nd _______ nd (+.002) (i.002) nd nd _______ nd 6 ____________________ 1.629 1.725 1.634 1.644 nd 1.616 (41002) (i 003) (i002) (i.002) _______ (i.002) 'y ____________________ 1.63 1.730 nd nd 181.628 (+.003) (11003) nd nd (i002) 2Vobs _______________ —~ 0° +~60° —-~0° nd —~90° 2Vcalc _______________ —17° +76° nd nd nd Cell data [Cell edges in angstroms; standard error in parentheses; interaxial angles in degrees, minutes, and fractions of stroms; molar volumes in cubic centimeters] a ____________________ 5.346 9.444 5.378 (41002) (i.028) (+.002) b ____________________ 9.21 5.520 9.320 (41008) (i.007) (+.004) c ____________________ 10.234 9.154 14.218 ("1005) (i 002) (42006) a ____________________ 0° 96°19 9’ ° _______ (:77) ______- fl ____________________ 100°10.9’ 101°53.5’ .7’ (i8.5') ($12.90 (i2.2’) 'y ____________________ 90° 90°53.7’ 90° _______ (i9.1’) ___---_ Cell volume __________ 496.2 463.8 707.4 (:3) ($1.3) (i.4) Molar volume ________ 74.7 34.9 106.5 (tJ) (i1) (i.1) Specific gravity calculated _________ 2.98 3.25 2.95 5.418 5.094 (+.001) (i.004) 9.385 _______ (41004) _______ 28.610 14.129 ($3002) (+5017) 96°54.7' 90° ( i1.1') _______ 90° 120° 717.6 317.5 (:3) (1‘5) 108.0 31.87 (i.1) (1.05) 2.99 4.61 atomic-absorption spectrophotometry; the standard solutions contained ap- proximately the same concentraton of H0104, sodium, and total iron pres- ent in the sample solution. FeO was determined volumetrically, the sample was decomposed by fusion with (NaF)2Ban in an atmosphere of argon; the melt was dissolved in an excess of standard potassium dichromate, and the excess dichromate was titrated with standard ferrous ammonium sul- fate in the presence of phosphoric acid using sodium diphenylamine-sul- fonate as indicator; the sample size was approximately 30 mg. Total water and 002 were determined simultaneously on a sample of approximately 200 mg using a microcombustion train; V205 was used as a flux; the value for H20“ was subtracted from the total water value to give 1120+. Approxi- mately 500 mg of sample was heated to constant weight at 110°i5° C to determine H201 6Reported values preceded by “less-than” signs (<) are not included in the sum. 7Anhydrous formula; 28.5 oxygen. aAnhydrous formula; 12 oxygen. 9Anhydrous formula; 11 oxygen. 1" Impure separate probably containing quartz. ‘1 Anhydrous formula; 6 oxygen. 121mpure separate probably containing chlorite and quartz; see micro- probe data (table 3, section D). 13 Anhydrous formula; 14 oxygen. 14 Impure separate. 15Anhydrous formula; 3 oxygen. 1“Anhydrous formula; 28 oxygen. Chemical analysis includes 0.21 percent fluorine. <0.01 percent chlorine; weight percent sum has been corrected for F-equivllent oxygen: number of atoms includes 0.090 fluorine. 17 H is calculated only for two staurolite samples and the tremolite sample. 15 Value is 7’ measured on fragment lying on the cleavage; Z’ A C:16°. 126 METAMORPHIC MINERAL ASSEMBLAGES, TACONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 5.—Compam'son of wet-chemical and microprobe analyses (A), in weight percent, and of the calculated chemical for- mulas (B) of staurolite from sample 355—1 A. Analyses [nd, not determined] W 6 melon”? Ra f Chemizal 1 Microprobe 2 ata $55,313 mg micropxi‘ig: data S102 ______________________ 28.515 28.08 28.08 27.73—28.48 TiOs ______________________ .41 . .62 .62 0.36— 0.77 IFklaga _____________________ 53.54 54.59 54.59 53.31—55.71 e2 3 _____________________ .54 2.61 FeO _______________________ 11.42 } ”-06 [11.71) 1330—1425 MnO _____________________ .08 .05 .05 0.02— 0.07 MgO _____________________ 1.06 1.05 1.05 0.99— 1.07 CaO ______________________ .09 .00 .00 0.00— 0.02 Na20 _____________________ nd nd nd nd K20 ______________________ <.0‘5 nd nd nd ZnO ______________________ .26 .35 .35 0.00— 0.77 H20+ _____________________ 1.61 (1.07) (1.07) __________ H20‘ _____________________ .02 nd nd nd C02 ______________________ <.02 nd nd nd P205 ______________________ .03 nd nd nd Sum __________________ 99.61 98.80 99.06 97.44-99.89 (99.87) (100.13) __________ B. Formulas [Fem total iron] 1. Formula of staurolite according to wet-chemical analysis, based on 23.5 oxygens for anhydrous formula: (F82 1.35Mg'o.ezMnomznomcaomFe3+o.1aTio.og)1.82 (Als.saFes+o.n) mooS isoaozsj (predicted H20+ is 1.06 weight percent); Fer/ (Few +Mg) 20.88. 2. Formula of staurolite according to microprobe analysis, both ferric and ferrous iron present [FeT/ (FeT+Mg)] :0.88: (Fez+1.87Mgo.22Mno.o1Zno.mFes+o_2eTio_a7)1.m(Als,ers+o,01)g,oo SEJZOZSJS 3. Formula. of staurolite according to microprobe analysis, only ferrous iron present: ( Fez.caMgo.22Mno.oerlo.MTio.m) 1.90A19.Msi‘3.95023.5 1 Analysis from table 4. that the anhydrous formula contains 23.5 oxygen atoms. Total iron is 2Average of analyses 1—5, table 3, section E, that were made by using the automated ARL—SMX probe of the US. Geological Survey in Reston, Va. H20+ figure in parentheses and sum in parentheses are calculated on the basis of a hydrous formula containing 24.0 oxygen atoms, assuming calculated as FeO. aTotal iron partitioned according the atomic ratio of the wet-chemical analyses. Figures in parentheses have the same meaning as do those in the preceding column. TABLE 6.—Molar volumes of minerals used to calculate slopes of the unlvariant curves shown in figure 15 Molar Mineral Formula volume Notes and sources (cmal Garnet ______ (F82+2.65Mgo.1scao.ao) Aleiaom 115.86 Robie and others, 1967; linear interpolation of end members. See also table 4. Staurolite ___ H(Fe“,Mg)zAIaSi4024 223.0 Robie and others, 1967; on natural sample of uncertain composition. See also table 4. _ Chloritoid ___ H2(Fe2*,Mg)Al-.Si07 69.6 Robie and others, 1967 ; on natural sample of uncertain composition. See also table 4. Chlorite _____ H3(F82+,Mg)4.5Alasi2.501s 208.0 Zen, 1972b, table 2. Biotite ______ H2K0.9(Fe2+1.5Mg1.w) A11.77Si2.e7012 152.5 Robie and others, 1967; linear interpolation of phlogo- pite and annite. Muscovite ___ H2KA13si3012 140.71 Robie and others, 1967. Anorthite ___- CaAlzsigos 100.79 Robie and others, 1967. Epidote _____ C39A12F€3+Si3013 138.7 Robie and others, 1967. Quartz ______ S102 22.688 Robie and others, 1967. 12 oxygen atoms for garnet. TABLES 127 TABLE 7.—Compositions of coexisting chlorite, biotite, TABLE 8.—Pa’rtial mineral-assemblage data for samples of chloritoid, and garnet in the presence of muscovite, epidote-beam'ng assemblages plagioclase, and quartz, in number of cations [Complete assemblage data are in table 1. +, mineral is present in assem- per formula 1 blage; —, mineral is absent from assemblage. Ga, garnet; Pa, paragonite; . Bt, biotite; Cd, chloritoid; Pg, plagioclase] [Figures are mean number of cations computed from data in table 3; figure in parentheses, standard deviation. Leaders in parentheses (____) indicate only one analysis was done for this mineral. Sample 339—1 Sample Ga Pa Bt Cd Pg contains chloritoid in garnet only; sample 466—1 does not contain garnet. Bdow garnet zone nd, not determined] 39—1 ______ _ _ :r _ + c 1:” 103—1 103—2 463—1 509-1 339—1 466—1 182-1 ------ - — — — ‘— 11 ion . 195_1 ______ _ _ + _ + Chlorltc 1916—1 ______ — — - — + 2.48 2.51 2.59 2.54 2.53 32%? """ _ + _ + + (.06) (.02) (.06) (.02) (.02) ‘ _ ------ _ '_ — _' _ .01 .00 .01 .00 .01 373—1 ______ —— — — — + (.01) (.01) (.00) (.01) (.01) 373_2 ______ _ _ _ _ + '70. 7-8:, is. 4-33, 7132. 376—1-2 -- — — — — 7332. 2:131. 2.132. 2.33;. 2.33:. 3..-... - - - - + :01 :01 I01 :00 :02 131:1 """ _ _ + _ i (.00) (.00) (.01) (.01) (.00) 1 ------ _ _ "" —' 73:. 1%?) 732. 73:. 172. 1934-2 ----- - - ‘-’ - + '1. 'nd nd nd '11.: 1103—2 """ " “' i : 1 nd nd nd nd nd — ------ _' _ nd nd _05 nd nd 1100—3 ______ — — — -—— + (.05) 1169—1 1 _____ — — ? — ? Biotite 1208—2 ------ — * — + 2.64 2.37 2.30) (2.68) (2.71) Garnet 1°“ .04 . 1) . 4 ___ _-_ (.10) (.09 .10 .10 .07 40—2 ______ — + — — + (.01) (.01) (.01) (.--) (___) 62—1 ______ + — — — — (17:. .172. 73;. 3-79. 3-84. 65—1—3 + — — — — 1.51 1246 1:33 1:25 1:46 67—2 —————— — + - — — (.03) (.05) (.07) (---) (___) 68—1 ______ + _ ._ + __ ('30) ('30) ('30) ( '01) ('01) 73—1 ------ + " — + + .89 I93 .94 1:07 :85 77—2 ______ + — — + + (.03) (.02) (.01) (___) (___) 77—3 ______ —- — — + + nd nd nd nd mi 131_]_ _ + _ + _ .04 .04 .03 .04 .06 , ------ (.01) (.01) (.01) (___) (___) 161—1 _____ + _ ._ _ + .85 .82 .82 .82 .69 170—1 3 _____ + — — — — (.07) (.03) (.04) (___) (-——) 170_23 _____ + _ _ __ Chloritoid 232-1 ------ + - + — + 346—1 ______ + — — — ? 1.00 1.00 1.01 1.00 1.00 506—2 ______ + _ + _ _ (.01) (.01) .01 .00 .01 nd nd (nd) (nd) (nd) 641—1 ------ _ — + “ + 1.98 2.00 1.98 1.98 1.98 . (.01) (.01) (.01) (.00) (.01) Staurolltc zone (517) (515) (5‘5 (515) (€17) 102 2‘ {-35 (~31. (‘33. (13%. (~31. 35611“ 31331 i 3 i 3 1 I14 :15 :15 I16 112 590—1 5 ----- + — + —- + (.00) (.01) (.01) (.00) (.01) nd nd nd nd nd 1 The biotite in samples 1014—2 and 1169—1 may be stilpnomelane. :3 nd “d “d “d 3 Contains hornblende. nd nd nd nd a , , , Contains cummingtonlte. Garnet 4 Dog not contain staurolite. . 5 Contains staurolite. SI ........... 2.97 2.97 2.98 2.99 2.99 (.01) (.01) (.03) (.02) (.02) Ti ___________ .00 .00 00 .00 .00 (.01) (.01) (00) (.00) (.00) Al ........... 2.01 1.99 1.99 2.00 2.00 (.02) (.02) ( 06) (.01) (.01) Fe .......... 2.48 2.34 47 2.37 2.45 (.07) (.03) ( 03) (.07) (.12) Mn .......... .25 .29 .25 .19 .14 (.04) (.01) ( 02) (.05) (.09) Mg __________ .16 .18 22 .18 .23 (.02) (.01) (0.8) (.03) (.02) Ca .......... .14 .25 11 .27 .20 (.02) (.02) (.01) (.03) (.05) Na .......... nd nd nd nd at! K ___________ nd nd nd nd nd 1Formula. computed on the anhydrous basis of 14 oxygen atoms for‘ chlorite, 11 oxygen atoms for biotite, 6 oxygen atoms for chloritoid, and) I 128 METAMORPHIC MINERAL ASSEMBLAGES, TAC‘ONIC ALLOCHTHON, MASS., CONN., N.Y. TABLE 9.—Compositions of coexisting minerals in three hornblende assemblages [The number of the sample analyzed is given below the mineral name. Figures are number of cations per formula calculated on the basis of anhy- drous versions of formulas given on p. 22-23; figures in parentheses, standard deviation. Compositions of hornblende from samples 102—1 and 289—2 are given by Doolan and others (1978). All other compositions are averages of data in table 3, parts A (muscovite), B (biotite), C (chlorite), F (garnet), and H (hornblende). nd, not determined. Sample 289—2 does not contain chlorite or muscovite; sample 161—1 does not contain biotite] C t' Hornblende Chlorite Garnet Biotite Muscovite 8. 10“ 102—1 161—1 289—2 102—1 161—1 102—1 161—1 289-2 102—1 289—2 102—1 161—1 Si __________ 6.08 6.06 6.09 2.62 2.54 2.94 3.05 2.96 2.65 2.73 3.05 3.02 (.12) (.07) (.11) (.03) (.11) (.01) (.05) (.01) (.07) (.03) (.00) (.06) T1 __________ .03 .0’2 .05 .01 .00 .01 .00 .01 .11 .12 .0‘1 .03 (.00) (.01) (.01) (.01) (.01) (.01) (.01) (.01) (.01) (.01) (.00) (.01) A1 __________ 3.27 3.42 3.14 2.75 3.13 1.98 1.94 1.98 1.68 1.59 2.84 2.88 (.27) (.14) (.16) (.06) (.19) (.02) (.02) (.01) (.01) (.05) (.01) (.10) Fe __________ 2.36 2.93 2.13 2.35 2.51 1.94 2.00 1.84 1.48 1.16 .08 .13 (.09) (.11) (.04) (.05) (.04) (.03) (.13) (.01) (.03) (.06) (.0'0) (.02) Mn _________ .03 .11 .04 .01 nd .36 .35 .43 .01 .01 .00 .00 (01) (10) (01) (.01) (.08) (.04) (07) (00) (00) ( 00) (00) Mg _________ 1 24 80 l 55 2.26 1.68 .16 .14 22 99 1 32 .07 (25) (03) (14) (.05) (.0‘8) (.02) (.01) (02) (01) (05) (.01) (01) Ca. __________ 1 73 1 65 1 77 .0'0 01 .68 .46 62 01 .00 .01 (08) (03) (03) (.01) (01) (.05) (.08) (04) (01) (00) (.00) (.00) Na _________ 35 40 00 00 nd .00 nd 02 02 .16 .09 (03) (03) (03 (.00) (01) (.00) (.01) (01) (.01) (.00) K ___________ . . .01 .09 nd .00 nd .00 nd .81 .88 .80 .72 (.00) (.01) (.01) (.00) (.00) (.02) (.01) (.01) (.00) TABLE 10,—Estimates of minimum temperature of metamor- phism based on K/ (K+Na) ratios of muscovite compositions and the 2.07-kbar solvus of Eugster and others (1972) and microprobe data [+, mineral is present: —, mineral is absent. Complete assemblage data are in table 1. K/(K+Na) ratios are based on data in tables 3, sec‘ tion A, and 4. Cd, chloritoid; St, staurolite; Ch, chlorite; Pa, paragonite] Esti- Sample Minerals associated with muscovite 01623.53; “$18531” No. Z°“°1 —"““‘—"“Cd s: Ch Pa K/(K ture, +Na) in ”C $50“ 140—2 ___ —-Ga + -- + — 0.80 540 360—1 -__ —Ga -—- — + + .79 550 509—1 ___ Ga + —- + —- .78 570 102—1 ___. Ga/St — — + — .84 480 838—1 __- Ga/St + -— + — .75 610 466—1 ___ Ga/St + — + — .74 620 506-1 __- Ga/St + + + — .72 640 . 103—1 _-_ St + — + — 81 5’0 103—2 ___ St + -- + — 83 500 331—1 ___ St + + + —- 69 670 339—1 ___ St —— —l- + — 73 640 355—1 ___ St — + + — 70 670 356—1 ..-- St — + + -- .74 620 369-1 ___ St + + + — .74 620 463-1 -_.. St + + + — .71 650 515—1 ___ St + + + -- .77 580 290-1 ___ +St — + + — .77 580 1052—1 ___ +St — + + —- .80 540 1 ——Ga, below garnet zone; Ga, garnet zone; Ga/St, at or near the boundary between the garnet and staurolite zones; St, staurolite zone: +St, high—grade part of the staurolite zone. Y} U.S. GOVERNMENT PRINTING OFFICE: |98| — 34l-6l4’137 PROCEDURES FOR ESTIMATING EARTHQUAKE GROUND MOTIONS High-altitude View of the San Framisco Bay region showing surface traces of the Hayward and (Ialaveras faults The Hayward fault was the source oftwo large earlhquakes in 1836 and 1858 that caused surface rupture along as much as 64 km of its trace. The Calaveras fault, with a length of approximately 160 km, is one of the largest in northern California. The San Andreas fault passes offshore near Mussel Rock (arrow at bottom of photog ‘aph). Fault systems such as these must be critically studied when evaluating the seismic hazards and risk in an urban area. Procedures for Estimating Earthquake Ground Motions By WALTER W. HAYS GEOLOGICAL SURVEY PROFESSIONAL PAPER 1114 UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1980 UNITED STATES DEPARTMENT OF THE INTERIOR CECIL D. ANDRUS, Secretary GEOLOGICAL SURVEY H. William Menard, Director Library of Congress Cataloging in Publication Data Hays, Walter W. Procedures for estimating earthquake ground motions. (Geological Survey professional paper ; 1114) Bibliography: p. 69-77. Supt. of Docs. no.: I 19.16:1114 1. Earthquakes—United States. 2. Seismology—United States. 3. Earthquake resistant design. I. Title. II. Series: United States. Geological Survey. Professional paper ; 1114. QE539.H39 363.3’495 79-607183 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, DC. 20402 ‘ Stock Number 024-001-03276-7 CONTENTS Page Page Glossary —————————————————————————————————————————————————— VII Define the characteristics of ground shaking—Continued Abstract __________________________________________________ 1 Spectra ________________________________________________ 34 Introduction_________---_-_-____________‘ ____________________ 2 Knowledge gained from nuclear explosion ground- Determine seismicity ______________________________________ 3 motion studies ________________________________________ 37 Sources of information __________________________________ 5 Intensity ______________________________________________ 41 Summary of United States earthquake history ____________ 5 Probabilistic estimates of peak ground acceleration ______ 42 Identify seismotectonic features ______________________________ 9 Effective peak ground acceleration ______________________ 43 Summary of United States earthquakes __________________ 12 Define design response spectra for site ______________________ 43 Western United States ______________________________ 12 Summary of procedures for siting of nuclear powerplants __ 44 Eastern United States ______________________________ 17 Site-independent response spectra ________________________ 46 Determine regional seismic attenuation ______________________ 19 Site-dependent response Spectra ________________________ 51 Define the characteristics of ground shaking expected at the site 21 DeSign time histories __________________________________ 54 The seismogram ________________________________________ 23 Determine local ground response ____________________________ 56 Types of information derived from the seismogram ________ 25 Summary of Uniform Building Code procedures __________ 63 Peak ground acceleration ________________________________ 28 Define uncertainties of the ground-motion design values ______ 66 Peak ground velocity and displacement __________________ 31 Seismic deSign trends for the future __________________________ 68 Duration ______________________________________________ 33 References cited ____________________________________________ 69 ILLUSTRATIONS Page FRONTISPIECE. High-altitude view of the San Francisco Bay region showing surface traces of the Hayward and Calaveras faults. FIGURE 1. Flow diagram illustrating steps in estimating ground motions for design of earthquake-resistant structures ______ 4 2—5. Maps showing: 2. Location of past destructive earthquakes in the United States __________________________________________ 6 3. Areas of the conterminous United States where regional seismicity studies have been made ______________ 7 4. Seismic source zones within the conterminous United States __________________________________________ 8 5. Principal faults in the vicinity of Fremont, Calif ______________________________________________________ 11 6. Chart for correlating fault rupture length and earthquake magnitude __________________________________________ 12 7-15. Maps showing: 7. Major tectonic features along the Pacific-North American plate boundary in Alaska ____________________ 12 8. Epicenters of earthquakes in Alaska between 1962 and 1969 __________________________________________ 13 9. Major faults in southern Alaska ____________________________________________________________________ 14 10. Major faults in California and Nevada and locations of past surface ruptures ____________________________ 15 11. Wasatch fault zone __________________________________________________________________________________ 17 12. Major tectonic features and historic earthquake activity in the Mississippi Valley area __________________ 18 13. Historic seismicity in South Carolina area ____________________________________________________________ 19 14. Isoseismal contours for 1971 San Fernando, Calif, earthquake ________________________________________ 20 15. Isoseismal contours for 1906 San Francisco and 1811 New Madrid earthquakes ________________________ 21 16—20. Graphs showing: 16. Empirical intensity attenuation curves proposed for the southern Appalachian seismic zone and the central Mississippi Valley ______________________________________________________________________ 22 17. Average value of maximum acceleration in relation to distance from fault for earthquakes of various magnitudes, Western United States ______________________________________________________________ 22 18. Schnabel and Seed acceleration-attenuation curves modified for use in the Eastern United States ________ 23 19. Acceleration-attenuation relations derived from worldwide earthquakes and the San Fernando earth- quake, 1971 ____________________________________________________________________________________ ‘ 24 20. Frequency-dependent attenuation of horizontal ground motions, southern Nevada and Colorado __________ 26 21. Schematic diagram of elements that affect ground motion ____________________________________________________ 26 22. Accelerogram of the 1940 Imperial Valley, Calif. earthquake recorded at El Centro, Calif ________________________ 27 V VI FIGURE 23. 24. 25. 26. 27. 28. 29. 30—35. 36. 37—41. 42. 43. 44—56. 57. 58. 59. 60. 61—64. 65. 67. CONTENTS Page S. 16° E. accelerogram recorded at Pacoima Dam and the velocity and displacement seismograms derived from it; 1971 San Fernando, Calif, earthquake __________________________________________________________________ 28 Graphic illustration of determination of Richter magnitude ____________________________________________________ 28 Graph showing range of horizontal peak acceleration as a function of distance and magnitude for rock sites in the Western United States __________________________________________________________________________________ 31 Graph showing relation between peak horizontal ground velocity and distance from source of energy release for magnitude 6.5 earthquakes ______________________________________________________________________________ 34 Bracketed duration values for the S. 16° E. accelerogram recorded at Pacoima Darn from the 1971 San F er- nando earthquake and graph showing bracketed duration as a function of magnitude and fault rupture length ________________________________________________________________________________________________ 35 Graph of integral definition of duration of shaking for a site on rock and a site underlain by alluvium ____________ 36 Graph of Fourier amplitude spectrum derived from the accelerogram recorded at El Centro from the 1940 Im- perial Valley, Calif. earthquake ________________________________________________________________________ 36 Schematic illustrations of: 30. Far-field displacement spectrum and some of the information about the source that can be derived from it ______________________________________________________________________________________________ 37 31. Narrow-band-pass filtering involved in deriving a response spectrum __________________________________ 38 32. Response spectra derived from the 1940 Imperial Valley, Calif. earthquake accelerogram ________________ 39 33. Time-dependent response envelope derived from the 1940 Imperial Valley, Calif. earthquake accelerogram 39 34. Source effects on the spectral composition of ground motion ____________________________________________ 39 35. Transmission path effects on the spectral composition of ground motion, 1971 San Fernando earthquake“ 39 Map showing Nevada Test Site and vicinity __________________________________________________________________ 4O Graphs showing: 37. Response spectra for 1940 Imperial Valley, Calif. earthquake and Cannikin nuclear explosion ____________ 4O 38. Response spectra for two sites equidistant from energy source but on different travel paths ______________ 40 39. Variability of ground motion recorded at two sites in Tonopah, Nev ____________________________________ 40 40. Intensity and acceleration relations __________________________________________________________________ 41 41. Mean values and standard-deviation error bars of peak ground acceleration, peak ground velocity, and peak ground displacement as a function of Modified Mercalli intensity, Western United States ______ 42 Map showing levels of peak horizontal ground acceleration expected at the 90-percent probability level at rock sites in the United States within a 50-year period ________________________________________________________ 43 Acceleration, velocity, and displacement seismograms from the 1966 Parkfield, Calif, earthquake recorded at station Cholame-Shannon No. 2 ________________________________________________________________________ 44 Graphs showing: 44. Variation in ground-motion response spectra and peak ground acceleration values for the same value of Modified Mercalli intensity, San Fernando, Calif, earthquake ____________________________________ 47 45. Site-independent velocity and acceleration response spectra ____________________________________________ 47 46. Schematic illustration of technique for developing site-independent response spectra ____________________ 49 47. “Standard” site-independent horizontal response spectra ______________________________________________ 50 48. Site-independent horizontal response spectra scaled to 1.0 g ____________________________________________ 51 49. Site-independent vertical response spectra ____________________________________________________________ 52 50. Comparison of site-independent, horizontal response spectra produced by three different procedures ______ 53 51. Site-dependent mean and mean-plus-one-standard-deviation response spectra for four site classifications -_ 54 52. Comparison of site-dependent mean and mean-plus-one-standard-deviation response spectra with AEC Regulatory Guide 1.60 spectrum ________________________________________________________________ 55 53. Effect of strain level on shear modulus and damping __________________________________________________ 56 54. Comparison of smooth response spectra for three soil columns __________________________________________ 57 55. Average acceleration response spectra for four site classifications ______________________________________ 57 56. Site transfer function for two sites in the Glendale, Calif. area derived from aftershocks of the 1971 San Fernando earthquake __________________________________________________________________________ 58 Map showing location of seismograph stations and thickness of alluvium in Las Vegas Valley and graphs showing variation of horizontal velocity response spectra for two stations and their site transfer function ____ 59 Contour maps showing the radial component of relative ground response in the period band 3.33—4.50 s and thick- ness of alluvium, Las Vegas Valley ______________________________________________________________________ 60 Variation of ground motion with depth, Beatty, Nev __________________________________________________________ 60 Parametric curves for amplification of SH waves ______________________________________________________________ 61 Graphs showing: 61. Example of site transfer function, elastic response ____________________________________________________ 62 62. Normalized rock response spectrum, 5-percent damping ______________________________________________ 62 63. Parametric curves for high-strain amplification of SH waves and three depths of unconsolidated materials ______________________________________________________________________________________ 63 64. Effect of peak acceleration level on amplification ______________________________________________________ 63 Map showing United States seismic risk zones ________________________________________________________________ 64 Map showing preliminary design regionalization proposed for 1976 Uniform Building Code ______________________ 65 Graph showing soil-structure interaction factor proposed for 1976 Uniform Building Code ______________________ 66 VII TABLES Page TABLE 1. Property damage and lives lost in notable United States earthquakes _________________________________________ 6 2. Summary of earthquake recurrence relations in the United States ___L _________________________________________ 7 3. Relative seismicity of regions of the United States ____________________________________________________________ 8 4. Seismicity parameters for seismic source zones ________________________________________________________________ 9 5. Criteria for recognizing an active fault ______________________________________________________________________ 10 6. Classification of fault activity-___________i-_______________--____________T ____________________________________ 11 7. Classification of selected faults relative to Fremont, Calif ______________________________________________________ 11 8. Relative intensity values and ground character, central California ____________________________________________ 19 9—13. Distance attenuation exponents derived from horizontal component PSRV spectra: 9. Northern Utah area ________________________________________________________________________________ 24 10. Southern Nevada area ______________________________________________________________________________ 25 11. California __________________________________________________________________________________________ 25 12. Piceance Creek Basin, Colo __________________________________________________________________________ 25 13. San Juan Basin, N. Mex _----_______-__________. ____________________________________________________ 25 14. Magnitudes and seismic moments of southern California earthquakes __________________________________________ 28 15. Values of peak horizontal ground acceleration recorded in past earthquakes and used for estimating horizontal ground motions in earthquake-resistant design __________________________________________________________ 29 16. Values of peak horizontal ground velocity and displacement derived from accelerograms of past earthquakes and used for estimating horizontal ground motions in earthquake-resistant design ______________________________ 32 17. Characteristics of the data samples used in selected studies of the correlation of Modified Mercalli intensity and peak ground acceleration ________________________________________________________________________________ 41 18. Horizontal ground accelerations for the operating basis earthquake and safe shutdown earthquake for nuclear powerplant sites in the United States ____________________________________________________________________ 46 19. Earthquake accelerograms used to derive site-independent spectra ___________________________________________ 48 20. Relative values of maximum ground acceleration, velocity, and displacement; “standard earthquake’ ____________ 50 21. Horizontal design response spectra and relative values of spectrum amplification factors for control points ________ 50 22. Vertical design response spectra and relative values of spectrum amplification factors for control points __________ 52 23. Uncertainties in physical parameters that affect ground motion ________________________________________________ 67 GLOSSARY Absorption. A process whereby the energy of a seismic wave is con- verted into heating of the medium through which the wave passes. Accelerogram. The record from an accelerometer showing acceler- ation as a function of time. Accelerometer. An instrument for measuring acceleration. Acceptable risk. A specification of the acceptable number of fatalities due to earthquake hazards, or an equivalent statement in terms of loss in buildings. Acoustic impedance. Seismic wave velocity multiplied by density of the medium. Active fault. A fault is active if, because of its present tectonic setting, it can undergo movement from time to time in the im- mediate geologic future. Aftershocks. Minor seismic tremors that may follow an under— ground nuclear detonation or the secondary tremors after the main shock of an earthquake. Alluvium. A general term for loosely compacted particles of rock, sand, clay. and so forth deposited by streams in relatively recent geologic times. Amplification. Modification of the input bedrock ground motion by the overlying unconsolidated materials. Amplification causes the amplitude of the surface ground motion to be increased in some range of frequencies and decreased in others. Amplification is a function of the shear-wave velocity and damping of the unconsoli- dated materials, its thickness and geometry, and the strain level of the input rock motion. Amplitude. Maximum deviation from mean or center line of wave. Amplitude spectrum. Amplitude versus frequency relation such as is computed in a Fourier analysis. See Fourier transform. Anisotrophic mass. A material having different properties in dif- ferent directions at any given point. Anisotropy. Variation of a physical property depending on the orientation along which it is measured. Anomaly. A deviation from uniformity or normality. Asthenosphere. The layer or shell of the earth below the litho- sphere; roughly equivalent to the upper mantle. Attenuation. (1) a decrease of signal amplitude during transmis- sion, (2) a reduction in amplitude or energy with or without change of waveform, or (3) the decrease in seismic signal strength with distance which depends not only on geometrical spreading but also may be related to physical characteristics of the transmitting medium causing absorption and scattering. Bandpass. Describing a range of frequencies (bandwidth) in which transmission is nearly complete while signals at frequencies out- side these limits are attenuated substantially. Bar. Equals 1 atmosphere: A unit of pressure, 0.01 kilopascal. Basement. The igneous, metamorphic, or highly folded rock under- lying sedimentary units. Bedrock. An solid rock exposed at the surface or underlying soil; has shear-wave velocity greater than 765 m/s at small (0.0001 percent) strains. See Firm soil and Soft soil. Body wave. Waves propagated in the interior of a body, that is, compression and shear waves, the P- and S-waves of seismology. Body-wave magnitude. mb. See Magnitude. Bulk modulus. The ratio of the change in average stress to the change in unit volume. Capable fault. A fault that has the potential to undergo future surface displacement. A fault is capable if: (1) it has had late VIII Quaternary or more recent movement, or (2) macroseimic activity has been associated with it, or (3) it has a demonstrated structural relation to a known capable fault such that movement of the one may cause movement of the other, especially during the lifetime of the project under consideration. Compression wave. A wave in which an element of the medium changes volume without rotation. Converted wave. A wave which is converted from longitudinal to transverse, or vice versa, upon reflection or refraction at oblique incidence from an interface. Convolution. The change of wave shape as a result of passing a signal through a linear filter. Corner frequency. A Spectrum’s comer frequency is that fre- quency where the high- and low-frequency trends intersect. The location of the corner frequency, )3, is related to the radius, r, of the equivalent circular fault through the relation r = %% where [i is the shear-wave velocity at the source. Covariance. A statistical regression analysis technique that allows one to analyze subsets of data having a common characteristic property. Critical angle. Angle of incidence, 06, for which the refracted ray grazes the surface of contact between two media in which the seis- mic velocities are U, and 02. Crust. The outermost portion of the earth, averaging about 30 km, that overlies the Mohorovicic discontinuity. Design earthquake. The largest earthquake that has such a high probability of occurrence based on studies of historic seismicity and structural geology that it is appropriate to design a structure to withstand it. Ground shaking of the design earthquake might be exceeded, but the probability of this happening is considered to be small. Design spectra. Spectra appropriate for earthquake-resistant de- sign purposes. Design spectra are typically smooth curves that have been modified from a family of spectra of historic earthquakes to take account of features peculiar to a geographic region and a particular site. Design spectra do not include the effect of soil- structure interaction. Design time history. One of a family of time histories which pro- duces a response spectrum that envelopes the smooth design spectrum, for a selected value of damping, at all periods. Earthquake hazards. The probability that natural events accom- panying an earthquake such as ground shaking, ground failure, surface faulting, tectonic deformation, and inundation, which may cause damage and loss of life, will occur at a site during a specified exposure time. See Earthquake risk. Earthquake risk. The probability that social or economic conse- quences of earthquakes, expressed in dollars or casualties, will equal or exceed specified values at a site during a specified expo- sure time. Earthquake waves. Elastic waves propagating in the earth, set in motion by a sudden change such as faulting of a portion of the earth. Effective peak acceleration. The peak ground acceleration after the ground—motion record has been filtered to remove the very high frequencies that have little influence upon structural response. Effective peak velocity. The peak ground velocity after the ground motion record has been filtered to remove high frequencies. Epicenter. The point on the Earth’s surface vertically above the point where the first rupture and the first earthquake motion oc- cur. Exceedance probability. The probability (for example, 10 percent) over some period of time that an earthquake will generate a level of ground shaking greater than some specified level. Exposure time. The period of time (for example, 50 years) that a structure is exposed to the earthquake threat. The exposure time is CONTENTS sometimes chosen to be equal to the design lifetime of the struc- ture. Failure. A condition in which movement caused by shearing stress- es in a structure or soil mass is of sufficient magnitude to destroy or seriously damage it. Fault. A fracture or fracture zone along which displacement of the two sides relative to one another has occurred parallel to the frac- ture. See Active, Capable, Normal, Thrust and Strike-slip faults. Filter. That part of a system which discriminates against some of ths information entering it. The discrimination is usually on the basis of frequency, although other bases such as wavelength may be used. Linear filtering is called convolution. Finite element analysis. An analysis which uses an assembly of finite elements which are connected at a finite number of nodal points to represent a structure or a soil continum. Firm soil. A general term for soil characterized by a shear wave velocity of 600 to 765 m/s. See Soft soil, Bedrock. Focal depth. The vertical distance between the hypocenter and the epicenter in an earthquake. Focus. The point within the earth which marks the origin of the elastic waves of an earthquake. Forced vibration. Vibration that occurs if the response is imposed by the excitation. If the excitation is periodic and continuing, the oscillation is steady-state. Fourier spectrum. See Amplitude spectrum, Phase response, and Fourier transform. Fourier transform. The mathematical formulas that convert a time function (waveform, seismogram, etc.) G(t) into a function of frequency S(f) and vice versa. Free field. The regions of the medium that are not influenced by manmade structures, or a medium that contains no such struc- tures. It also refers to that region in which boundary effects do not significantly influence the behavior of the medium. Free vibration. Vibration that occurs in the absence of forced vibration. Frequency. Number of cycles occurring in unit time. Hertz (hz) is the unit of frequency. Gaussian distribution; Equals normal distribution (bell-shaped curve): A quantity or set of values so distributed about a mean value, m, that the probability, 6(Aa), of a value lying within an interval, Aa, centered at the point, a, is: €(Aa)=_1 e __—(a—m)2 Aa 27117 2 where o- is the standard error of estimate. Geometrical damping. That component of damping due to the ra- dial spreading of energy with distance from a given source. Geophone. Sensing device used to measure electronically the rate of travel of sound or force waves transmitted through the earth from a known source. Geophysics. The study of the physical characteristics and prop- erties of the earth. Geotechnical. Related to soil mechanics. Grain size. A term relating to the size of mineral particles that make up a soil deposit. Ground response, ground motion, seismic response. A general term, includes all aspects of ground motion, namely, particle accel- eration, velocity, or displacement; stress and strain from a nuclear explosion, an earthquake, or another energy source. Group velocity. The velocity with which most of the energy in a wave train travels. In dispersive media where velocity varies with frequency, the wave train changes shape as it progresses so that individual wave crests appear to travel at a different velocity (the phase velocity) than the overall energy as approximately enclosed by the envelope of the wave train. CONTENTS IX Half-space. A mathematical model bounded only by one plane sur- face; that is, the model is so large in other dimensions that only the one boundary affects the results. Properties within the model are assumed to be homogeneous and usually isotropic. Hertz. A unit of frequency; cycles per second (cps). Holocene. The past 10,000 years of geologic time. Hydrostatic pressure. The pressure in a liquid under static condi- tions; the product of the unit weight of the liquid (water=1 kg/L) and the difference in elevation between the given point and the ground water elevation. Hypocenter. The location in space where the slip responsible for an earthquake occurs; the focus of an earthquake. Hysteresis loop. (1) the stress-strain path of a material under cyc- lic loading conditions, or (2) a trace of the lag in the return of an elastically deformed specimen to its original shape after the load has been released. Incident angle. The angle which a ray path makes with a perpen- dicular to an interface. In situ strength. The in-place strength of a soil deposit. Intensity. A numerical index describing the effects of an earth- quake on the earth’s surface, on man, and on structures built by him. The scale in common use in the United States today is the Modified Mercalli scale of 1931 with intensity values indicated by Roman numerals from I to XII. The narrative descriptions of each intensity value are: I. Not felt or, except rarely under especially favorable circum- stances. Under certain conditions, at and outside the bound- ary of the area in which a great shock is felt: sometimes, birds, animals, reported uneasy or disturbed; sometimes dizziness or nausea experienced; sometimes trees, struc- tures, liquids, bodies of water, may sway; doors may swing, very slowly. II. Felt indoors by few, especially on upper floors, or by sensitive, or nervous persons. Also, as in grade I, but often more noticeably: sometimes hanging objects may swing, espe- cially when delicately suspended; sometimes trees, struc- tures, liquids, bodies of water, may sway, doors may swing, very slowly; sometimes birds, animals, reported uneasy or . disturbed; sometimes dizziness or nausea experienced. III. Felt indoors by several, motion usually rapid vibration. Some- times not recognized to be an earthquake at first. Duration estimated in some cases. Vibration like that due to passing of light, or lightly loaded trucks, or heavy trucks some dis- tance away. Hanging objects may swing slightly. Movements may be appreciable on upper levels of tall struc- tures. Rocked standing motor cars slightly. IV. Felt indoors by many, outdoors by few. Awakened few, espe— cially light sleepers. Frightened no one, unless apprehen- sive from previous experience. Vibration like that due to passing of heavy or heavily loaded trucks. Sensation like heavy body striking building or falling of heavy objects in- side. Rattling of dishes, windows, doors; glassware and crockery cli'nk and clash. Creaking of walls, frame, espe- cially in the upper range of this grade. Hanging objects swung, in numerous instances. Disturbed liquids in open vessels slightly. Rocked standing motor cars noticeably. V. Felt indoors by practically all, outdoors by many or most; out- doors direction estimated. Awakened many, or most. Frightened few—slight excitement, a few ran outdoors. Buildings trembled throughout. Broke dishes, glassware, to some extent. Cracked windows—in some cases, but not gen- erally. Overturned vases, small or unstable objects, in many instances, with occasional fall. Hanging objects, doors, swing generally or considerably. Knocked pictures against wall, or swung them out of place. Opened, or closed, doors, shutters, abruptly. Pendulum clocks stopped, started or ran fast, or slow. Moved small objects, furnishings, the later to slight extent. Spilled liquids in small amounts from well- filled open containers. Trees, bushes, shaken slightly. VI. Felt by all, indoors- and outdoors. Frightened many, excite- ment general, some alarm, many ran outdoors. Awakened all. Persons made to move unsteadily. Trees, bushes, shaken slightly to moderately. Liquid set in strong motion. Small bells rang—church, chapel, school, etc. Damage slight in poorly built buildings. Fall of plaster in small amount. Cracked plaster somewhat, especially fine.cracks in chim- neys in some instances. Broke dishes, glassware, in consid— erable quantity, also some windows. Fall of knick—knacks, books, pictures. Overturned furniture in many instances. Moved furnishings of moderately heavy kind. VII. Frightened all; general alarm, all ran outdoors. Some, or many, found it difficult to stand. Noticed by persons driving motor cars. Trees and bushes shaken moderately to strongly. Waves on ponds, lakes, and running water. Water turbid from mud stirred up. Incaving to some extent of sand or gravel stream banks. Rang large church bells, etc. Sus— pended objects made to quiver. Damage negligible in build- ings of good design and construction, slight to moderate in well-built ordinary buildings, considerable in poorly built or badly designed buildings, adobe houses, old walls (especially where laid up without mortar), spires, etc. Cracked chim— neys to considerable extent, walls to some extent. Fall of plaster in considerable to large amount, also some stucco. Broke numerous windows, furniture to some extent. Shook down loosened brickwork and tiles. 'Broke weak chimneys at the roof-line (sometimes damaging roofs). Fall of cornices from towers and high buildings. Dislodged bricks and stones. Overturned heavy furniture, with damage from breaking. Damage considerable to concrete irrigation ditches. VIII. Fright general; alarm approaches panic. Disturbed persons driving motor cars. Trees shaken strongly—branches, trunks, broken off, especially palm trees. Ejected sand and mud in small amounts. Changes: temporary, permanent; in flow of springs and wells; dry wells renewed flow; in temper- ature of spring and well waters. Damage slight in structures (brick) built especially to withstand earthquakes. Consider- able in ordinary substantial buildings, partial collapse: racked, tumbled down, wooden houses in some cases; threw out panel walls in frame structures, broke off decayed pil- ing. Fall of walls. Cracked, broke, solid stone walls seri- ously. Wet ground to some extent, also ground on steep slopes. Twisting, fall, of chimneys, columns, monuments, also factory stacks, towers. Moved conspicuously, over- turned, very heavy furniture. IX. Panic general. Cracked ground conspicuously. Damage con- siderable in (masonry) structures built especially to with- stand earthquakes: Threw out of plumb some wood-frame houses built especially to withstand earthquakes; great in substantial (masonry) buildings, some collapse in large part; or wholly shifted frame buildings off foundations, racked frames; serious to reservoirs; underground pipes sometimes broken. X. Cracked ground, especially when loose and wet, up to widths of several inches; fissures up to a yard in width ran parallel to canal and stream banks. Landslides considerable from river banks and steep coasts. Shifted sand and mud horizon- tally oh beaches and flat land. Changed level of water in wells. Threw water on banks of canals, lakes, rivers, etc. Damage serious to dams, dikes, embankments. Severe to well-built wooden structures and bridges, some destroyed. Developed dangerous cracks in excellent brick walls. De- stroyed most masonry and frame structures, also their foun- dations. Bent railroad rails slightly. -Tore apart, or crushed endwise, pipe lines buried in earth. Open cracks and broad wavy folds in cement pavements and asphalt road surfaces. XI. Disturbances in ground many and widespread, varying with ground material. Broad fissures, earth slumps, and land slips in soft, wet ground. Ejected water in large amounts charged with sand and mud. Caused sea-waves (“tidal” waves) of significant magnitude. Damage severe to wood- frame structures, especially near shock centers. Great to dams, dikes, embankments often for long distances. Few, if any (masonry) structures remained standing. Destroyed large well-built bridges by the wrecking of supporting piers, or pillars. Affected yielding wooden bridges less. Bent rail- road rails greatly, and thrust them endwise. Put pipe lines buried in earth completely out of service. XII. Damage total—practically all works of construction damaged greatly or destroyed. Disturbances in ground great and var- ied, numerous shearing cracks. Landslides, falls of rock of significant character, slumping of river banks, etc., numer- ous and extensive. Wrenched loose, tore off, large rock masses. Fault slips in firm rock, with notable horizontal and vertical offset displacements. Water channels, surface and underground, disturbed and modified greatly. Dammed lakes, produced waterfalls, deflected rivers, etc. Waves seen on ground surfaces. Distorted lines of sight and level. Threw objects upward into the air. Interface. The common surface separating two different geologic media in contact. Internal friction. The resisting shear strength considered to be due to the interlocking of the soil grains and the resistance to sliding between the grains. Isotropic. Having the same physical properties regardless of the direction in which they are measured. Strictly applies only to an arbitrarily small neighborhood surrounding a point and to single properties. Lame’s constants. See Modulus. Least squares fit. An analytic function which approximates a set of data with a curve such that the sum of the squares of the distances from the observed points to the curve is a minimum. Linear system. A system whose output is linearly related to its input. If a linear system is excited by a an input sine wave of frequency 7“,, the output will contain only the frequency f,; the amplitude and phase may be changed. Linear viscoelastic medium. A medium for which the functional relation between stress and strain can be expressed as a linear relation between stress, strain and their nth order temporal de- rivatives. Liquefaction Temporary transformation of unconsolidated mate- rials into a fluid mass during an earthquake. Lithology. The description of rock composition and texture. Lithosphere. The crust and upper mantle of the earth. LoadEgTThe force on an object or structure or element 6? a strucT ture. Longitudinal wave. Equals compression wave; equals P-wave. Love wave. A seismic surface wave which propagates in a surface layer. The vibration is transverse to the direction of propagation with no vertical motion. Low-cut filter. Equals high pass filter: A filter that transmits fre- quencies above a given cutoff frequency and substantially at- tenuates lower frequencies. Low-pass filter. Equals high-cut filter: A filter that transmits fre- quencies below a given cutoff frequency and substantially atten- CONTENTS . uates all others. The earth acts like a low-pass filter. Magnitude. A quantity that is characteristic of the total energy released by an earthquake, as contrasted to intensity, which sub- jectively describes earthquake effects at a particular place. Profes- sor C. F. Richter devised the logarithmic magnitude scale in current use to define local magnitude (ML) in terms of the motion that would be measured by a standard type of seismograph located 100 km from the epicenter of an earthquake. Several other mag- nitude scales are in use, for example, body-wave magnitude (mb) and surface-wave magnitude (M,) which utilize body waves and surface waves. The scale is open ended, but the largest known earthquake magnitudes (M,) are near 8.9. Major principal stress. (See Principal stress) The largest (with regard to sign) principal stress. Mantle. The part of the earth’s interior between the core and the crust. The upper surface of the mantle is the Mohorovicié dis- continuity. , Meizoseismal zone. Zone of intense shaking in the near field of an earthquake. The radial width of this zone increases with mag- nitude. Mesosphere. The lower mantle. It is not involved in the earth’s tectonic processes. Microtremor. A weak tremor. Minor principal stress. (See Principal stress). The smallest (with regard to sign) principal stress. Model. A concept from which one can deduce effects that can then be compared to observations; this concept assists in understanding the significance of the observations. The model may be conceptual, physical , or mathematical. Modulus. A measure of the elastic properties of a material. Moduli for isotropic bodies include: 1. Bulk modulus, k. The stress-strain ratio under simple hydro- static pressure: the bulk modulus can be expressed in terms of other moduli as: k=E/3(1—20'). 2. Shear modulus. Equals rigidity modulus, equals Lame’s con- stant, [1.2 The stress-strain ratio for simple shear. The shear modulus can also be expressed in terms of other moduli and Poisson’s ratio 0' as: E ”z 2(1+0') ' 3. Young’s modulus, E. The stress-strain ration when a rod is pulled or compressed. 7 4. Lame’s constant A. If a cube is stretched in the up—direction by a tensile stress, S, is giving an upward strain, .9, and S ’ is the lateral tensile stress needed to prevent lateral contraction, then: AtSVs . This constant can also be expressed in terms of Young’s modulus, E, and Poisson’s ratio, a: = E 0' (1+0) (1—20). The velocities of P- and S-waves, V,, and V_,-, can be expressed in terms of the moduli and the density, p: V, = \/()\+2;L)/p VI,- = Viz/p . Modulus of elasticity. The ratio of stress to strain for a material CONTENTS XI under given loading conditions; numerically equal to the slope of the tangent of a stress-strain curve. Mohoroviéié (M) discontinuity. Seismic discontinuity which sep- arates the earth’s crust and mantle. Moment. The seismic moment M,,=;uZA contains information on the rigidity (M) in the source region, average dislocation (L2), and area (A) of faulting. It determines the amplitude of the long-period level of the spectrum of ground motion. , Monotonic loading. Continuously increasing load in one direction. Natural frequency. Property of the elastic system in free vibra- tion. Free vibration occurs naturally at a discrete frequency when an elastic system vibrates under the action of forces inherent in the system itself and in the absence of external impressed forces. Normal fault. Vertical movement along a sloping fault surface in which the block above the fault has moved downward relative to the block below. The Wasatch fault in Utah is an example. Normal stress. That stress component normal to a given plane. Operating basis earthquake (OBE). A design earthquake used by the U.S. Nuclear Regulatory Commission in nuclear power plant siting: the largest earthquake that reasonably could be expected to affect the plant site during the operating life of the plant. The powerplant is designed to withstand the OBE and still operate without undue risk to the health and safety of the public. See Safe shutdown earthquake. Oscillation. The variation, usually with time, of the magnitude of a quantity with respect to a specified reference when the magnitude is alternately greater and smaller than the reference. Overburden. The generic term applied to uppermost layers of the geologic structure, usually unconsolidated materials having low seismic velocity overlying rock. P-wave. (See also Compression or Body wave). Body wave in which the direction of the particle motion is the same as the direc- tion of wave propagation. P-wave velocity is commonly measured in geophysical refraction surveys to define the contact between the competent rock layers (high-velocity materials) and the overlying unconsolidated materials (low-velocity materials). Particle acceleration. The time rate of change of particle velocity. Particle displacement. The difference between the initial position of a soil particle and any later position. Particle velocity. The time rate of change of particle displacement. Period. The time interval occupied by one cycle. Permeability. A measure of the ease with which a fluid can pass through the pore spaces of a formation. Phase. The angle of lag or lead (or the displacement) of a sine wave with respect to a reference; the stage in the course of a rotation or oscillation to which it has advanced, considered in relation to a reference or assumed instant of starting. Phase response. A graph of phase shift versus frequency illus- trates the phase response characteristics of a system. The amplitude-frequency response of a filter to the shape of pulses put through it will be different for different phase characteristics; this response leads to phase distortion. Phase velocity. The velocity with which any given phase (such as a wave of single frequency) travels; it may differ from group veloc- ity because of dispersion. Plane strain (biaxial). A measure of strain that takes place in two directions while remaining zero in the third dimension. Plastic range. The stress range in which a material will not fail when subjected to the action of a force, but will not recover com— pletely, so that a permanent deformation results when the force is removed. Plate tectonics. A theory introduced in 1967 and subsequently re- fined that considers the earth’s crust and upper mantle to be made up of more than 15 relatively undistorted plates about 60 km thick which move relative to one another. The plates spread from the mid-oceanic ridges where, by means of the upflow of magma, new lithospheric material is continually added. On the opposite mar— gins of the plates, there are usually deep submarine trenches. At these trenches, the plates converge from opposite directions (for example, the Nazca and South American plates along the Andes Mountains), and one plate is consumed or subducted beneath the other into the deeper parts of the earth. Earthquake belts or zones mark plate boundaries, the zones along which the lithospheric plates collide, diverge, and slide past one another. The San An- dreas fault zone is an example of a boundary between the North American and Pacific plates. Poisson’s ratio. The ratio of the transverse contraction to the lon- gitudinal extension when a rod is stretched. The ratio of the veloci- ties of P- and S-waves, VP and Vs, can be expressed in terms of Poisson’s ratio, 0': V,, = 2(1-0) W (1—2cr>' Pore water pressure. Pressure or stress transmitted through the pore water filling the voids of the soil. Power spectrum. A graph of power spectral density versus fre- quency. The power spectrum is the square of the amplitude- frequency response. Principal stress. Stresses acting normal to three mutually perpen- dicular planes intersecting at a point in a body, on each of which the shearing stresses are zero. Probability of occurrence. The annual rate of occurrence of a hazard. Pulse. A waveform whose duration is short compared to the time scale of interest and whose initial and final values are the same (usually zero). Q. Q, the reciprocal of the specific dissipation function, is an index of the dissipative nature of the earth’s transmission path on prop- agating seismic waves. Q is essentially independent of frequency or wavelength for a wide range of frequencies. Empirically deter- mined values of Q range from 50 to 500 for crustal materials in various regions of the United States. Also called quality factor. Rayleigh wave. A type of seismic surface wave which propagates along the surface. Particle motion is elliptical and retrograde in the vertical plane containing the direction of propagation, and its amplitude decreases exponentially with depth. Raypath. A line everywhere perpendicular to wavefronts in iso- tropic media. The path which a seismic body wave takes. Reflection. The energy or wave from a seismic source which has been reflected (returned) from an acoustic impedance contrast or series of contrasts within the earth. Reflection coefficient. The ratio of the amplitude of a reflected wave to that of the incident wave. For normal incidence on an interface which separates media of densities pI and p2 and veloci- ties V, and V2, the reflection coefficient for a plane wave is: P2V2 _ P1V1 P2V2 + prI Region. A geographical area surrounding and including the site sufficiently large to contain all the features related to a physical phenomenon or to a particular earthquake hazard. Relative density. The ratio of (1) the difference between the void ratio of a cohesionless soil in the loosest state and any given void ratio, to (2) the difference between its void ratios in the loosest and in the densest state. Resonance. The reinforced response of one of the natural modes of vibration of a body when excited at a frequency close to the natural frequency of vibration. Resonant frequency. A frequency at which resonance occurs. XII Response. The motion in a system resulting from an excitation under specified conditions. Response spectrum. The peak response of a series of simple har- monic oscillators of different natural period when subjected math- ematically to a particular earthquake ground motion. The re- sponse spectrum may be plotted as a curve on' tripartite logarithmic graph paper showing the variation of the peak spectral acceleration, displacement, and velocity of the oscillators as a function of vibration period and damping. Return period. The average period of time or recurrence interval between events causing ground shaking exceeding a particular level at a site; the reciprocal of annual probability of exceedance. A return period of 475 years means that, on the average, a particular level of ground motion will be exceeded once in 475 years. Risk. See Earthquake risk. Rock. See Bedrock. Rupture velocity. The velocity at which the fault rupture propa- gates along its length. The rupture velocity is usually some frac- tion of the shear wave velocity and in the range 1.5 to 4.5 km/s for most earthquakes. It affects the effective stress. S-wave (Shear wave). Body wave in which the particle motion is at right angles to the direction of wave propagation. SH and SV denotes planes of polarization of wave. S-wave velocity may be measured by in-hole geophysical procedures to determine the dynamic shear moduli of the materials through which the wave passes. Safe shutdown earthquake (SSE). A design earthquake used by the US. Nuclear Regulatory Commission in nuclear powerplant siting: the largest possible earthquake at the site, considering the regional and local geology and seismology and specified charac- teristics of local and subsurface material. Important nuclear power plant structures, systems, and components are designed to remain intact during the SSE. See Operating basis earthquake. Saturated soil. Soil with zero air voids. A soil which has its in- terstices or void space filled with water to the point where runon occurs. Seismic source zones. Areas of spatially homogeneous earthquake activity. Seismic wave. An elastic wave generated by an earthquake or ex- plosion which causes only a temporary displacement of the medium, the recovery of which is accompanied by ground vibrations. Seismogram. A record of ground motion or of the vibrations of a structure caused by a disturbance, such as an underground nuclear detonation or an earthquake. See Accelerogram. Seismog'raph. A system for amplifying and recording the signals from seismometers. Seismometer. The instrument used to transform seismic wave enegry into an electrical voltage. Most seismometers are velocity detectors their outputs being proportional to the velocity of the inertial mass with respect to the seismometer’s case (which is pro- portional to the velocity of the earth motion.) Below the natural frequency, the response of most geophones decreases linearly with frequency so that they operate as accelerometers. Seismotectonic province. A geographic area characterized by similarity of' geologic structure and earthquake characteristics. Sensitivity. The smallest change in a quantity that a detector can detect. Shear modulus (Rigidity). The ratio between shear stress and shear strain in simple shear. Shear wave. Equals S-wave equals transverse wave: A body wave in which the particle motion is perpendicular to the direction of propagation. Shear wave velocity. See Modulus. Simple shear. The state of stress at the point where only shearing stresses act on any two perpendicular planes. CONTENTS Site vicinity. The geographical region within about 10-km radius of the center of the site. Snell’s law. When a wave crosses a boundary, the wave changes direction such that the sine of the angle of incidence divided by the velocity in the first medium equals the sine of the angle of refrac- tion divided by the velocity in the second medium. Soft soil. A general description for soil which has a shear wave velocity less than 600 m/s. See Firm soil, Bedrock. Sonic log. A record of the seismic velocity (or of interval time) as a function of depth. Specific dissipation. See Q. Standard deviation. The standard deviation, 0', of n measure- ments of a quantity x,, with respect to the mean, X, is: a: ( (X,—X)2) 1/2 Standing wave. A wave produced by simultaneous transmission in opposite directions of two similar waves. It may result in fixed points of zero amplitudes called nodes. Station. A ground position at which a geophysical instrument or seismograph is set up for an observation. Steady-state vibration. Vibration in a system where the velocity of each particle is a continuing periodic quantity. Stochastic. Random; value determined entirely by chance. Strain. The change in length per unit of length in a given direction which a body subjected to deformation undergoes. Strain dependent property. A property exhibited by soil wherein the magnitude of a physical property depends on the magnitude of the induced strain. Stratigraphy. The order of succession of the different sedimentary rock formations in a region. Strength of earthquake. In current usage, it is often expressed in terms of the peak ground acceleration recorded or predicted for a particular earthquake, expressed usually in g units, where 1 g is an acceleration equal to that of gravity, or 980 cm/sz. Stress drop. Adam—o. where 00 is the initial stress before the earthquake and 0', is the stress after the earthquake. For the 1971 San Fernando, Calif. earthquake, the average initial stress is es- timated to have been about 100 bars and the stress drop to have been about 60 bars. Stress drop controls the high-frequency spectral content of earthquake ground motions. Stress (effective). In modeling an earthquake, the effective stress is defined as 0:00—07 where (To is the stress before the earthquake and 0', is the frictional stress acting to resist the fault slip. Strike-slip fault. A fault in which movement is principally horizon- tal. The San Andreas fault is strike-slip. Strong motion. Ground motion of sufficient amplitude to be of en- gineering interest in the evaluation of damage due to earthquakes or nuclear explosions. Structural features. Features such as faults and folds which are produced in rock by movements after deposition, and commonly after consolidation, of the constituent sediment. Surface waves. Seismic energy which travels along or near the surface. Rayleigh and Love waves. Surface wave magnitude. MS, See Magnitude. Tectonic province. As defined by the US. Nuclear Regulatory Commission, it is a region of the North American continent characterized by the uniformity of the geologic structures con- tained therein. Tectonic structure. As defined by the US. Nuclear Regulatory Commission, it is a large dislocation or distortion within the earth’s crust whose extent is greater than several kilometers. Tectonics. A branch of geology dealing with the broad architecture of the upper part of the earth’s crust in terms of the origin and 1 t N—1 i=1 CONTENTS historical evolution of regional structural or deformational fea- tures. Thrust fault. An inclined fracture along which the rocks above the fracture have apparently moved up with respect to those beneath. The 1964 Alaska and 1971 San Fernando earthquakes occurred on thrust faults. Time-distance curve. Equals T—X curve: A plot of the arrival time of refracted events against the source-receiver distance. Time dependent. Describing an operation in which the physical parameters vary with time. Transferfunction. Filter characteristics in the frequency domain as represented by the amplitude-versus-frequency and phase angle-versus-frequency curves. Contains the same information as the impulse response in the time domain and is convertible into the impulse response through the Fourier transform. Transform. To convert information from one form into another, as with the Fourier transform. Transverse wave. See Shear wave. Upper-bound earthquake. The hypothetical earthquake that is considered to be the most severe reasonably possible on the basis of comprehensive studies of historic seismicity and structural geology. Vibration. An oscillation. The motion of a mechanical system. Viscoelastic. Having a stress-strain relation that includes terms proportional to both the strain and the rate of strain. Leads to frequency-dependent attenuation for seismic waves. Materials with this property are also called Voigt solids. XIII Viscoelastic medium. A stress-strain relation in which the stress is a function of both strain and strain rate, although not necessar- ily proportional to both. Viscosity. The cohesive force between particles of a fluid that causes the fluid to offer resistance to a relative sliding motion between particles; internal fluid friction. Viscous damping. The dissipation of energy that occurs when a particle in a vibrating system is resisted by a force that has a magnitude proportional to the speed of the particle and a direction opposite to the direction of the particle. Void ratio. The proportion of void space in a given soil mass. Voig‘t solid. See Viscoelastic. Waveform. A plot of voltage, current, seismic displacement, etc., as a function of time. Wave guide. A geologic situation which permits surface waves. Wave length. Normal distance between two wave fronts with peri- odic characteristics and a phase difference of one complete cycle. Wavelet. A seismic pulse usually consisting of 1% to 2 cycles. Wavenumber. The number of wave cycles per unit distance; recip- rocal of wavelength. White noise. Random energy containing all frequency components in equal proportions. . Young’s modulus. E. The ratio of unit stress to unit strain within the elastic range of a material under a given loading condition, numerically equal to the slope of the tangent of a stress-strain curve. PROCEDURES FOR ESTIMATING EARTHQUAKE GROUND MOTIONS By WALTER W. HAYS ABSTRACT This paper is a comprehensive review of the current procedures used for specifying earthquake ground motion for many applications in the United States. These applications include: siting and design of nuclear powerplants, hospitals, dams, schools, oil pipeline systems, waste storage facilities and military facilities; city and land-use planning; disaster planning; building codes; and evaluation of insur- ance needs for indemnification of losses from natural hazards. The seismic design problem varies considerably in the United States be- cause of the markedly different seismicity in the east and west. In the Western United States, particularly in California and Nevada, the seismicity is very high. More large- and moderate-sized earth- quakes have occurred in these two states than in any other part of the conterminous United States. Tectonic surface ruptures related to historic (0—200 years before presentland Holocene (0—10,000 years before present) earthquakes are common. In addition, the active plate boundary represented by the 1,000-km-long San Andreas fault system makes part of the Western United States very different from other seismically active parts of the country. By contrast, the Central and Southeastern United States have almost none of the characteris— tics exhibited in the Western United States. Seismicity is low, large- to moderate—sized earthquakes occurred only in 1811, 1812, and 1886, and no evidence of tectonic surface ruptures related to historic or Holocene earthquakes has been found. Also, no currently active plate boundaries of any kind are known there. Setting the seismic design parameters for a site requires consid- eration of a large body of geologic, geophysical, seismological, and geotechnical information. For the region surrounding the site, statis- tical and deterministic models are developed from this information to characterize the geologic province where the earthquake occurs, the earthquake source and wave propagation path, and the local ground response. Knowledge of the physical parameters that affect the characteris- tics of ground motion and their range of values has been gained from: observational and instrumental data from earthquakes, nuclear ex- plosions and aftershock sequences, regional seismicity networks, geologic mapping, trenching of fault zones, analytical models, geophysical measurements, and laboratory measurements. Although each source of data has contributed to the knowledge about ground motion, all these data have not been incorporated into earthquake- resistant design procedures. For example, current procedures do not use stress drop or seismic moment. Also, conservatism is introduced in the current procedures because the various physical processes that occur during an earthquake are not completely understood, and statistical distributions for many empirical relations used to esti- mate earthquake ground motions are not well defined. Seven basic steps are followed in specifying the characteristics of the ground motion needed for earthquake-resistant design. They are: 1. to determine the seismicity of the geographic region where the application is planned, 2. to identify the seismotectonic features, 3. to estimate the regional seismic attenuation, '4. to estimate the ground shaking parameters (for example, peak acceleration or Modified Mercalli intensity) at the site, 5. to define the ground-motion response spectra for the site, 6. to determine the local amplification effects and to modify the design response spectra for the site as necessary, and 7. to estimate the uncertainty in the ground-motion design values. Each step requires careful evaluation of the best available data. For some applications, the available data may be inadequate for precise specification of the earthquake ground motion. In these cases, an effort is generally made to use ground motion values that, based on experience and the uncertainty in the data, are considered to be conservative. Examples of conservatism include: using a rare event as the maximum possible earthquake because the location and mag- nitude of potential earthquakes are uncertain, using upper-bound values for the peak ground acceleration expected at a site because of uncertainties in the regional seismic attenuation function and the local ground response, and using upper-bound values for the design response spectrum because the details of ground-motion spectra can vary widely for a given value of peak ground acceleration or Modified Mercalli intensity. The controversy that exists today in earthquake- resistant design is largely a debate over whether the geologic, geophysical, seismological, and geotechnical data are adequate for the specific design application under consideration and whether the judgments about conservative ground-motion estimates are rea- sonable. The ideal data base, which is not yet available for all geographic regions of the United States, should contain the following informa- tion for the region surrounding the site: 1. Seismicity A complete historical record and location map of all reported earthquakes in the region; Information about the source parameters such as epicenter, focal depth, source mechanism and dimensions, magnitude, stress drop, effective stress, seismic moment, and rupture velocity) of each historic earthquake; Earthquake-recurrence relations for the region and for specific seismic source zones in the region. 2. Seismotectonic features Maps showing the seismotectonic provinces and capable faults; , Information about the earthquake potential of each seis- motectonic province including information about the geometry, amount, sense of movement, and temporal his- tory of each fault and the correlation with historical and instrumental earthquake epicenters; Correlation of historic earthquakes with tectonic models to estimate the upper-bound magnitude (or seismic moment) that should be associated with specific tectonic features. 1 2 PROCEDURES FOR ESTIMATING EARTHQUAKE GROUND MOTIONS 3. Seismic attenuation Isoseismal maps of significant historic earthquakes that oc- curred in the region; Strong ground—motion records of historic earthquakes; Scaling relations and their statistical distribution for ground-motion parameters as a function of distance. 4. Characteristics of historic ground shaking Isoseismal maps of all significant historic earthquakes that have affected the site; Ensembles of strong ground-motion records of earthquakes that occurred either in the region of interest or in other regions that have similar source—path-site characteristics adequate for “calibrating” the near—field, the regional seismic wave attenuation characteristics, and the local ground response. 5. Earthquake spectra Ensembles of spectra (Fourier, power spectral density, and response) adequate for “calibrating” the near-field, the transmission path, and the local ground response. 6. Local amplification effects Seismic-wave transmission characteristics (amplification or damping) of the unconsolidated materials overlying bed- rock and their correlation with physical properties, includ- ing seismic shear wave velocities, bulk densities, shear moduli, strain levels, water content, and geometry; Strong ground motion records at surface and subsurface loca- tions for a wide range of strain levels. Very limited empirical data exist for the near field of earthquakes. Also, analytical models are presently considered to be inadequate for specifying the details of the ground motion in a manner that is ac- ceptable in current earthquake-resistant design. In the near field, ground-motion parameters are strongly influenced by the dynamics of the fault rupture, a physical process that is not well understood at the present time. The strong-motion accelerograph network is the source of ground-motion date used in earthquake-resistant design. About 200 of the approximately 500 digitized records currently available are basement or free-field records, principally from the 1971 San Fer- nando, California earthquake. Only a few records have been ob- tained in the near field distance range in which earthquakes have caused significant damage. Relatively few records have been ob- tained at surface and subsurface sites underlain by rock and uncon- solidated materials having varied physical characteristics. Also, only accelerograms from small earthquakes have been recorded in the Eastern United States. Better estimates of earthquake ground motion will require im- proved data. Regional seismicity and strong-motion accelerograph networks must be expanded in order to obtain adequate data for “calibrating" large regions of the United States in terms of their earthquake source mechanisms, regional seismic attenuation, and local ground response. Better geologic and geophysical data and analyses are needed to define the earthquake potential and upper- bound magnitude in different geographic regions and to establish the dynamics of faulting and recurrence intervals for specific faults. Knowledge about the origin of intraplate earthquakes, which seem to have different causes than the earthquakes that occur along plate boundaries, is needed to assess more accurately the earthquake po- tential of low-seismicity regions such as the Eastern United States. INTRODUCTION This paper is a part of ongoing investigations by the US. Geological Survey to better understand the causative mechanisms and physical effects of earth- quakes in order to contribute to the reduction of earth- quake hazards in the United States. This paper pro- vides a comprehensive summary of current procedures that use geologic, geophysical, seismological, and geotechnical data for estimating ground-motion characteristics of earthquakes for sites of interest in the United States. These procedures are used in many applications, including siting of nuclear power plants (US. Atomic Energy Commission, 1971 and 1973 a, b; American Society of Civil Engineers, 1976); construc- tion of hospitals (Veterans Administration, 1973); con- struction of dams (Peak, 1973); seismic design of oil pipeline systems (Page and others, 1972; Newmark and Hall, .1973); construction of schools (State of California, 1933); construction of military facilities (Department of Army, Navy and Air Force, 197 3); city and land-use planning (California Council on Inter- governmental Relations, 1972 and 1973; Woodward- NcNeill and Associates, 1973; Nichols and Buchanan-Banks, 1974); city lifeline engineering (American Society of Civil Engineers, 1974); regional earthquake zonation (Borcherdt, 1975; Espinosa, 1977); earthquake ground shaking hazard maps (Algermissen, 1969; Algermissen and Perkins, 1976); disaster planning (Office of Emergency Preparedness, 1972, Algermissen and others, 1972, 1973; Hopper and others, 1975, Rogers and others, 1976); insurance against natural hazards (Baker, 1971); building codes (Wiggins and Moran, 1971; Applied Technology Coun- cil, 1976), and management of hazardous wastes (Healy and others, 1968;Wyss and Molnar, 1972). A great deal of important research is being per- formed through the earthquake research programs of the US. Geological Survey (USGS) and the National Science Foundation (National Science Foundation and US. Geological Survey, 1976; Hamilton, 1978). These programs are complementary and represent a balanced study of six elements: (1) fundamental earthquake studies, (2) earthquake prediction, (3) induced seismic- ity, (4) earthquake hazards assessment, (5) engineer- ing, and (6) research for utilization. Academic and private-sector workers are involved in these studies along with USGS personnel. The information contained in this paper is developed for the geologist or engineer who needs to know how to estimate the earthquake ground motion for a site. It represents several scientific disciplines and is a subset of the comprehensive body of knowledge now available. This subset of information provides a framework for a technical understanding of the empirical procedures currently used to estimate ground motion. Simplifications and generalizations are made to some degree in the paper to enable the geologist or engineer to have a broad understanding of the techni— cal concepts without having to know all of the details. DETERMINE SEISMICITY 3 Extensive references are provided to give additional information when needed. Also, a glossary of of stan- dard nomenclature, terminology, and definitions is provided to aid understanding. Some subjects, such as the causative mechanisms of US. earthquakes, are ob- viously too complex to be covered completely; there- fore, only the most important facts related to the prob- lem of estimating earthquake ground-motion are em- phasized. Earthquake-resistant design is a dynamic field; therefore, the procedures discussed in this paper should be expected to change as progress in fundamen- tal research is made.1 A number of investigators (for example, Barosh, 1969; Lomenick, 1970; Werner, 1970; Shannon and Wilson, Inc., and Agbabian Associates, 1972, 1975; Algermissen, 1973; Hoffman, 1974; Trifunac, 1973b; Hays and others, 1975b; Krinitzsky and Chang, 1975; Werner, 1975, Gates, 1976; Guzman and Jennings, 1976; American Society of Civil Engineers, 1976; and McGuire, 1977a) have published procedures for es- timating earthquake ground motions. On the basis of accumulated knowledge and experience in geology, geophysics, seismology, and geotechnical engineering, a general procedure, such as shown in figure 1, pro- vides a reasonable basis for specifying the ground- motion parameters needed for earthquake-resistant design. _ Empirical correlations are widely used today in earthquake resistant design. These correlations are frequently based on statistical analysis of the best available data, but, in many cases, they are based on interpreted trends or projected upper bounds in the data. Thus, all the empirical correlations do not have well-defined statistical distributions. The relations most frequently used are: 1. Earthquake recurrence relations (Algermis- sen, 1969; Algermissen and Perkins, 1976); 2. Magnitude and Modified Mercalli intensity (Gutenberg and Richter, 1942); 3. Peak ground acceleration and Modified Mer- calli intensity (Neumann, 1954; Gutenberg and Richter, 1956; Trifunac and Brady, 1975c; O’Brien and others, 1977); . Magnitude and length of fault rupture (Bonilla, 1970; Wallace, 1970; Housner, 1970; Mark, 1977); 5. Peak acceleration, distance, and magnitude (Housner, 1965; Schnabel and Seed, 1973; Donovan, 1973; Boore and others, 1978); 6. Modified Mercalli intensity and distance (Nut- tli, 1972; Brazee, 1976; Young, 1976); 'Note added in press. The October 15, 1979, Imperial Valley, Calif, earthquake is an example ofa significant opportunity to increase our knowledge. This ML 66 earthquake was well recorded, especially near the fault, and will provide a basis for improving present procedures used in earthquake-resistant design. A 7. Duration of shaking and magnitude (Housner, 1965; Page and others, 1972; Bolt, 1973; Trifunac and Westermo, 1977); 8. Peak velocity and distance (Esteva and Rosenblueth, 1964; Seed, Murarka, Lysmer, and Idriss, 1976); 9. Site-independent response spectra (Housner, 1959; Newmark and Hall, 1969; US. Atomic Energy Commission, 1973; N. M. Newmark Consulting Engineering Services, 1973; and J. A. Blume and Associates, En- gineers, 1973); 10. Site-dependent response spectra (Seed, Ugas, and Lysmer, 1976). Deterministic models have been developed by a number of investigators (for example, Brune, 1970; Schnabel, Lysmer and Seed, 1972) to simulate numeri- cally the various physical processes that occur in an earthquake. With the exception of the local ground- response models, deterministic models generally have not been adopted in current earthquake-resistant de- Slgn. Probabilistic methods have recently emerged as a useful way to estimate ground motion. It will take a number of years, however, to evaluate their full range of usefulness and to incorporate them into earth- quake-resistant design. The steps in a general procedure for estimating earthquake ground motion for earthquake-resistant design are discussed below. DETERMINE SEISMICITY The objective of this step is to use the historic seis- micity record to define earthquake recurrence relations applicable for the region of interest, to provide a basis for correlating earthquake epicenters and tectonic structure, and to define seismic source zones. From the seismicity record, one attempts to establish: 1. The date of occurrence of each past event; 2. Epicentral intensity and, for post-1933 events, magnitude; 3. Epicenter and hypocenter locations; 4. Epicentral maps showing the epicenters of all re- ported earthquakes centered within an 80-km radius of the site and all earthquakes with Modified Mercalli intensity greater than or equal to V within a 320—km radius; The aftershock zone of historic earthquakes; Correlation of epicenter locations and tectonic , structure; 7. The frequency of occurrence for earthquakes of various magnitudes or epicentral intensities. Evaluation of the seismicity of a region requires the 9M" PROCEDURES FOR ESTIMATING EARTHQUAKE GROUND MOTIONS USER PROVIDES SITE AND STRUCTURE INFORMATION Steps PREVIOUS EARTHQUAKE DATA AVAILABLE? DATA 1 ANALYZED ESTIMATE SEISMICITY OBTAIN AND ON BASIS OF ANALYZE DATA EXISTING ' “56'0”“- DATA DETERMINE SEISMICITY PARAMETERS 0F AREA |—> 3 < J 2 IDE:'TIFY,SEISMDTECTDNIc FEATURES OF AREA 3 I ESTIMATE SEISMIC ATTENUATIDN FOR AREA I * USER DEFINES 4 ESTIMATE MAXIMUM INTENSITY ACCEPTABLE LEVEL 0 F RISK OF SHAKING EXPECTED FOR SITE T 5 ESTIMATE GROUND MOTION RESPONSE SPECTRA EXPECTED FOR SITE USER PROVIDES CRITERIA IS STRUCTU RE CRITICALLY IMPORTANT? DAMAGE OR DESIGN 6 ESTIMATE LOCAL SOIL AMPLIFICATION EFFECTS MODIFY RESPONSE SPECTRA FOR SITE (IF NECESSARY) I ESTIMATE UNCERTAINTY IN SEISMIC DESIGN PARAMETERS DOES CRITICAL NATURE OF STRUCTURE REOUIRE ADDITIONAL ANALYSIS? Yes USER PROVIDES ANALYSIS CRITERIA PERFORM DETAILED SITE AND STRUCTURE ANALYSIS H—_‘ I —+I I t USER DOCUMENTS ANALYSES, ALTERNATIVES, AND RECOMMENDATIONS FIGURE 1.—Steps in estimating ground thion for design of earthquake-resistant structures. DETERMINE SEISMICITY 5 compilation and evaluation of an earthquake catalog (Stepp, 1972; Gardner and Knopoff, 1974; Kagen and Knopoff, 1976). This task is complex because the de- sign application (for example, a dam, hospital, or nu- clear power plant) under consideration may require a fairly precise specification of the frequency of occur- rence of large earthquakes on a local scale whereas the catalogs of instrumentally recorded earthquakes and felt earthquakes generally do not cover a time interval long enough to allow valid extrapolations of future earthquake activity except on a broad regional scale. It has been clearly demonstrated by Evernden (1970) and others that the earthquake recurrence law follows the empirical linear relation log N(M)=a—bM , where N (M ) is the number of earthquakes occurring within a region in a given time period with a mag- nitude greater than or equal to M. The constants a and b are determined from least squares analysis. The seismicity index a is dependent upon the size of the geographic area, the level of activity, and the length of the time period considered. The seismic severity index b is usually in the range -0.8 to —1.0 for most parts of the world and appears to be related to the nature of the tectonic activity causing the earthquakes. A similar empirical linear relation holds for Modified Mercalli intensity. SOURCES OF INFORMATION There are numerous sources of seismicity data. The most complete single source of seismicity data is Earthquake History of the United States (Coffman and Von Hake, 1973) which is published by NOAA (Na- tional Oceanic and Atmospheric Administration). In addition, detailed information is given in the annual publication, U .S. Earthquakes, formerly published by US. Department of Commerce but now published jointly by NOAA and USGS. This publication contains reproductions of important accelerograms, isoseismal maps, and other important data. A catalog of some 6,000 US. earthquakes is con- tained in Hays and others (1975b). This catalog in- cludes earthquakes through 1970 that have been as- signed Modified Mercalli intensities of IV or greater. Information about the seismicity in individual states is usually available through the State Survey or a state university. For example, information on the seismicity of Nevada can be obtained from the University of Nevada, Reno, and from publications such as Rogers, Perkins, and McKeown (1976) and Ryall (1977). Infor- mation on the seismicity of Utah is available from the University of Utah at Salt Lake City. Townley and Allen (1939) cataloged preinstrumental data in California, whereas information on the instrumental seismicity of California can be obtained from Califor- nia Institute of Technology’s Seismological Laboratory, University of California, Berkeley’s Seismic Station, and the California Division of Mines and Geology. In- formation on Alaska’s seismicity is contained in the publication by Meyers (1976). Recently, an earthquake information center was established at Memphis State University, Tennessee. The seismicity of the Eastern United States has been discussed by several investigators (for example, Mc- Clain and Meyers, 1970; Bollinger, 1972, 1973; Chin- nery and Rogers, 1973; Sbar and Sykes, 1973; Hadley and Devine, 1974; Nuttli, 1974; Long, 1974; Young, 1976, and McGuire, 1977b). The Bulletin of the Seismological Society of America contains data about recent earthquakes in the "Seis- mological Notes” of each issue. Current data are frequently available through the National Earthquake Information Service, USGS, Golden, Colo. (for example, Stover and others, 1976). The earthquakehistory of the United States will be discussed below to illustrate the problem of seismicity on a national scale. Regional earthquake recurrence relations, “b values,” upper-bound magnitudes, and seismic source zones can be readily compared on this scale. SUMMARY OF UNITED STATES EARTHQUAKE HISTORY Although the zone of greatest seismicity in the United States is along the Pacific Coast in Alaska and in California, the central and eastern parts of the United States have also experienced seismic activity (fig. 2). Earthquakes have occurred in the St. Lawrence River region on many occasions from 1650 to 1928, in the Boston vicinity in 1755, in the central Mississippi Valley at New Madrid, M0,, in 1811—1812, in Charles- ton, S.C., in 1886, and at Hebgen Lake, Mont., in 1959. Historical earthquakes that have caused notable damage and loss of life are listed in table 1. Property damage in the United States due to earthquakes occur- ring since 1865 approaches $2 billion. The loss of life in the United States has been relatively light, consider- ing the number of destructive earthquakes that have occurred. Since 1964, the United States has experi- enced one great earthquake (the 1964 Alaska earth- quake). The Alaska earthquake had a magnitude of 8.4 and caused $500 million in property damage, 131 deaths, and hundreds of injuries. The San Fernando earthquake had a magnitude of 6.6. It released about one-thousandth as much energy as the Alaska earth- quake, but it caused the same amount of property 6 PROCEDURES FOR ESTIMATING EARTHQUAKE GROUND MOTIONS 120° 110° 100° 90° 30° 60° I I I I I I I I \ \ \ \ 1965 St. Lawrence River region, 1949 Puget Sewn-1663, 1870 1 y T i . ' / \ ~. I' I Helena, 1935 Imvskazggg . K-~\ ....»\- \— . Manhattan area, I '. x) t .1925 . x '~ He en Lake, : .- / \. .Mohhgn, 1959 I‘ — ----------- -I (" . I \, East of Cape .' '\. J‘\~ ....... ‘ .~ ‘ \ 1 Ann, 1755 - I I t L. r)" -\.‘ .' , . --—r‘ \..\__\_/ Kosrno, Utah, I L ________ A ’_ \ I, ‘ / 40° \ 111““?va 1934 '_ ......... \ ‘- 7 ‘ --""' \ )191-5 I . "I I "‘I. . _,.—-- . N y k '1‘ L I I \ Anna \ 1;; W or I San FranCIsco, : "‘-- ..‘ . '~ " ' 1906‘ I I I‘ "“"~—. I f I 2.1937 (L, , ‘ '1 Hayward, I: ‘ I l_ ___________ S ----- V i i. ,-/ ‘ 1868 \ Owens Valley, .Elsnnore,; : K. \x I\ N 1872 ’92 7 I I \. I, Giles County, San .\ -‘ " ' ' I J" } ..v'a 1897 I~F d ~. /‘-\__ L I Charleston,-. /"' ,1 ‘- " .’-‘/ / ernan 0 . Kern C0., . \~\__ .. .' ‘, / .. ,./>-’ . 1971 .1952 ~. ,_. , ‘HHML ______________ __l Mo.,1895 _________ ’0’ T J 88 , _\I I .__ ___ = ,._.,_... L—f / ma Barbara. RIVBYSIde C0,, 1918 , I j New Madrid .- '. {/- L 7925 .Hemet. 7899 ' ’ I 1 ' .y _____ .L--7’ \"\"“‘~ ong Beach, 1933 . I I _ lMo. 1.91142 ,__ ......... \ .. Imperial Valley, . » / I . \ San Juan .1940 - \_ : x . ' ‘\ Capistrano, 1812 I I 'V‘xu—hrnd I I \-_ “\ Charleston, I I I ..... -/._ i \ '- 7886 / : _\ _____ __,-' K ’I I ( .Big Bend area, 1. / 3 _' “.11 _ _H 1931 ) ‘----‘-\ l ” ‘ L I I | I I - ’0 H A w A _ / W g “51> 1973 o 500 KILOMETERS Kane. 1951 |_1_1_1_;_l I I l l 1 78581 I I l l 1 \ FIGURE 2.—Location of past destructive earthquakes in the United States. TABLE 1,—Property damage and lives lost in notable United States earthquakes [From Office of Emergency Preparedness, v. 3 (1972), p, 80—82] Damage (million Lives Year Locality Magnitude dollars) lost 1811—12----New Madrid, M0. 7.5 (est) __,- _--- 1865 ______ San Francisco, Calif. 8.3 (est.) .4 -___ 1868 ______ Hayward, Calif. ____ .4 30 1872 ______ Owens Valle , Calif. 8.3 (est.) .3 27 1886 ______ Charleston, .C. __-_ 23.0 60 1892 ______ Vacaville, Calif. ____ .2 ---- 1898 ______ Mare Island, Calif. ____ 1.4 -___ 1906 ______ San Francisco, Calif. 8.3 (est) 500.0 700 1915 ______ Imperial Valley, Calif. -_-_ 6.0 6 1925 ...... Santa Barbara, Calif. ____ 8.0 13 1933 ...... Long Beach, Calif. 6.3 40.0 115 1935 ...... Helena, Mont. 6.0 4.0 4 1940 ______ Imperial Valley, Calif. 7.0 6.0 9 1946 ______ Hawaii (tsunami) ____ 25.0 173 1949 ...... Puget Sound, Wash. 7 1 25.0 8 1952 ______ Kern County, Calif. 7 7 60.0 8 1954 ...... Eureka, Calif. ____ 2.1 1 1954 ...... Wilkes-Barre, Pa. ____ 1.0 ---- 1955 ______ Oakland, Calif. -___ 1.0 1 1957 ______ Hawaii (tsunami) -___ 3.0 _--- 1957 ______ San Francisco, Calif. 5 3 1.0 ”h 1958 ______ Khantaak Island and Lituya Bay, Alaska ____ ---- 5 1959 ______ Hebgen Lake, Mont. ---- 11.0 28 1960 ______ Hilo, Hawaii (tsunami) ____ 25.0 61 1964 ______ Prince William Sound, Alaska 8.4 500.0 131 1965 ______ Puget Sound, Wash. ___- 12.5 7 1971 ...... San Fernando, Calif. 6.6 553.0 65 damage and half as many deaths because the San Fer- nando earthquake occurred on the edge of a large met- ropolitan area, whereas the Alaska earthquake oc- curred in a sparsely populated region. One indication of the seriousness of the earthquake problem in the United States is the fact that earth- quakes were felt in 34 states in 1973. All or parts of 39 states lie in regions classified as having major and moderate seismic risk. Within these 39 states, more than 70 million people are exposed to earthquake hazards. The focal depths of earthquakes vary widely throughout the United States. Earthquakes in Califor- nia occur at relatively shallow depths, (S16 km), whereas large earthquakes in the Puget Sound, Wash- ington, area occur primarily at depths of 50—60 km. Earlier, scientists thought that earthquakes in the central United States (for example, New Madrid area) occurred at depths of 50—60 km, but recent studies have shown that they occur at depths of 5—20 km. A shallow depth of 5—10 km is also indicated for earth- quakes in the Charleston, SC. area. The historical seismicity record in the United States is not more than 400 years long and quite variable regionally. Earthquake recurrence relations have been derived from this relatively short record. (Algermissen, DETERMINE SEISMICITY 7 1969). Figure 3 shows the regions studied by Algermis- sen in 1969 to determine earthquake recurrence rela- tions, expressing the number of earthquakes, N, occur- ring in a given region in a given time period with intensity greater than or equal to Modified Mercalli intensity I. These recurrence relations are summarized in table 2. These data, integrated with Alaska seismic- ity data, provide the relative ranking of regional seis- micity shown in table 3. In 1976, Algermissen and Perkins defined 71 regions (fig. 4) within the conterminus United States as seis- mic source zones. Each zone was defined on the basis of the historic seismicity and the distribution and activity of faults. Table 4 lists the seismicity parameters de- TABLE 2,—Summary of earthquake recurrence relations in the United States [From Algermissen (1969), fig. 3. N, number of earthquakes of intensity I] Earthquakes/ 100 yrs/ 100,000 km2 for Area Recurrence relation Modified Mercalli Nos. from Intensity fig' 3 5m“ v VI VII VIII 1 California ______________________________________ log N=3.92—0.54 I 300 84.6 23.8 6.72 2 Nevada ________________________________________ log N=3.98—0.56 I ________________________ 3 Puget Sound, Wash ______________________________ log N=3.45—0.62 I 68.0 16.3 3.92 94 4 Montana, Idaho, Utah, Arizona (Intermountain Seismic Belt) ____________ log N=3.41—0.56 I 64.4 17.7 4.99 1.35 5 Wyoming, Colorado, New Mexico ____________________________________ log N=3.66—0.68 I 32.8 6.85 1.42 31 6 Oklahoma, North Texas __________________________________________ log N=2.10—0.55 I 13.3 3.73 1.07 30 7 Nebraska, Kansas, Oklahoma ______________________________________ log N: 1.99—0.49 I 13.0 4.20 1.35 45 8A, Mississippi and St. B Lawrence Valleys ____________________________ log N = 2.7 1—0.5 I 24.2 7.65 2.42 76 9 East Coast ______________________________________ log N=3.02—0.58 I 12.8 3.39 .88 23 / 3 / ‘1 .i I y ( l €' ----- I 40" \"\‘/'\ ..... I ». l i o l “' ‘ . I 3° i 'F “““ "x I I ~ ' \ 4/ \ 1 L...— — """"""" " \ s f/ o 500 KILOMETERS ~ |—_l—_.|._.l_.l_J ,. 120° 110° 100° 90° 80° FIGURE 3.—Geog'raphic areas of the conterminous United States where regional seismicity studies have been made (modified from Algermissen, 1969). Numbered areas are described in table 2. Shading depicts boundaries of regions. 8 PROCEDURES'FOR ESTIMATING EARTHQUAKE GROUND MOTIONS TABLE 3.—Relatiue seismicity of regions of the United States Equivalent magnitude-4 earthquakes/year/ Normalized to Area 1,000 km2 Pacific West S.E. Alaska (lat 54°—63° N.) (long 144°—156° W.) ________ 15 75 Pacific West (west of long 114° W.) ,,,,,, .20 10 Rocky Mountains (long 106°— 114° W.) ____________ .04 .20 Central Plains (long 92°—106° W.) ,,,,,,,,,, .006 .03 Eastern United States (east of long 92° W.) ________ .017 .08 termined for each source zone. On the basis of current knowledge, future earthquakes are considered equally likely to occur anywhere in a source zone. Estimating the upper-bound magnitude for various geographic regions of the United States is a difficult problem (Smith, 1976). The nature of the problem var- ies for different areas, as discussed below, and its solu- tion depends upon comprehensive studies of the avail- able geologic and seismological data: 95" 1. Area of high seismicity, several major shocks, and a fairly long (>100 years) historical rec- ord of earthquakes—The number of major (M>8) shocks may be insufficient to obtain a reliable average rate of occurrence, but the average rate of occurrence can probably be estimated reasonably well from a plot of log N =a—bM . California and Alaska are exam- ples of such an area. 2. Area of high seismicity, some moderate shocks, no known major shocks, fairly long (>100 years) historical record of earthquakes—The crustal rocks may not be able to support high levels of strain and thus strain accumulation is relieved through moderate magnitude (6—7) shocks. The rate of occurence of moderate and small shocks can probably be estimated accurately from the historic record. Unless geologic evidence indicates the possi- bility of large shocks, it is probably justified to assume that no large shocks will occur. The Puget Sound area and the Intermountain 75° 70 65° 125° 120° 115° 110° 105° 100° V 21 22 A 23 '28: 45° 0 20 24 l- @ , ~ 22 L. 2 _ ; /__ I a: 23 3 1M @) I '. 26 ‘I a N ______________ 3‘ -. '9 ~ . ._ l‘ '3 . / 43 - \'\" zé I l 40a 1 " ' ~- 0 . 41 42 : 48 7 1o . f— —. ._ . 33 43 l 5 '7 ' ~\-~_ . 10 , ----- a . 9 32 32 3 . . , 39 o 3 7 -.11 ,- 34 I l 35 \m 2 7 ‘2 x " ‘ a 43 43 50 3 14 r» I ~» .. _§_ _ _ 4 15 33 ......... 3’ ‘6 44 : I 5 52 2 \ 13 17 / I 32 7 l 19 as l 30° / 43 : 45 25° cuLF 0F MEXICO 0 115° 110° 105° 1oo 95° 90° 85° 80° 75° FIGURE 4.—Seismic source zones within the conterminous United States (from Algermissen and Perkins, 1976). Zone numbers correspond to those in table 4. IDENTIFY SEISMOTECTONIC FEATURES 9 TABLE 4.—Seismicity parameters for seismic source zones [Zone numbers shown in fig. 4] Number of Modified Seismic Corresponding Zone Mercalli maximum severity Maximum maximum No. intensity V’s/100 yrs index b intensity magnitude 1 ______ 245.2 —O.50 X 7.3 2 ...... 110.0 —.40 up to IX then zero XII 8.5 3 ______ 27.2 —.45 XI 7.9 4 ______ 75.1 —.45 XI 7.9 5 ______ 14.9 —.50 X 7.3 6 ______ 44.4 -.45 XI 7.9 7 ______ 299.6 —.53 VIII 6.1 8 ...... 7.3 —.49 VI 4.9 9 ______ 208.0 —.40 XI 7.9 10 ______ 125.0 —.51 VIII 6.1 11 ______ 80.1 —.53 VIII 6.1 12 ______ 43.0 —.43 up to XI then zero XII 8.5 13 ______ 99.4 —.45 XI 7.9 14 ______ 34.9 -.45 XI 7.9 15 ...... 0.0 —.53 VIII 6.1 16 ______ 33.9 —.50 X 7.3 17 ...... 223.0 -.45 XI 7.9 18 ______ 2.8 —.50 X 7.3 19 ...... 613.6 -.52 X 7.3 20 ______ 14.8 —.29 VIII 7.1 21 111111 79.8 —.59 VII 5.5 22 ______ 80.1 —.76 VI 4.9 23 ______ 12.7 Not applicable V 4.3 24 ______ 6.0 __ 0 __________ V 4.3 25 ______ 8.5 —.59 VII 5.5 26 ______ 137.1 —.72 VI 4.9 27 ______ 99.9 —.67 VII 5.5 28 ______ 35.3 —.32 IX 6.7 29 ______ 90.4 —.36 X 7.3 30 ______ 10.5 —.26 VII 5.5 31 ______ 84.6 —.63 VII 5.5 32 ______ 17.0 ~.56 V1 4.9 33 ______ 126.8 —.56 IX 6.7 34 ______ 71.0 —.56 VII 5.5 35 ______ 23.0 —-.56 VIII 6.1 36 ...... 15.3 —.54 VII 5.5 37 ...... 15.6 —.31 VIII 6.1 38 ______ 31.1 —.54 VII 5.5 39 ______ 21.5 —.54 VII 5.5 40 ...... 2.7 —.40 VI 4.9 41 ______ 27.6 Not applicable V 4.9 42 ______ 11.1 —.40 V1 4.9 43 ______ 23.0 Not applicable V 4.3 44 ______ 13.8 __ 0 __________ V 4.3 45 ______ 6.7 —.31 VIII 6.1 46 ______ 2.7 —.40 VI 4.9 47 ______ 2.7 —.40 VI 4.9 48 ______ 14.7 -.54 VII 5.5 49 ______ 10.3 Not applicable V 4.3 50 ______ 4.6 __ o __________ V 4.3 51 ,,,,,, 7.4 —.53 VI 4.9 52 ______ 13.0 -.40 VI 4.9 53 ______ 9.3 —.24 VIII 6.1 54 ______ 21.2 -.55 VII 55 55 ...... 1.7 Not applicable V 4.3 56 ______ 5.7 —.53 VI 4.9 57 ______ 7.8 —.55 VII 5.5 58 ...... .6 —.50 VII 5.5 59 ______ 16.0 —.50 VIII 6.1 60 ______ 16.0 —.50 VIII 6.1 61 ______ 84.5 —.50 X 7.3 62 ______ 22.0 —.50 VIII 6.1 63 ______ 22.1 —.64 VIII 6.1 64 ______ 54.4 —.59 VIII 6.1 65 ...... 19.9 —.33 X 7.3 66 ______ 13.0 —.59 VIII 6.1 67 ______ 7.8 —.59 VII 5.5 68 ______ 69.1 —.67 VIII 6.1 69 ______ 117.6 —.59 IX 6.7 70 ...... 33.5 —.65 VIII 6.1 71 ______ 21.7 —.49 X 7.3 Seismic Belt are examples of this type of area. 3. Area of moderate seismicity and occasional or single occurrence of a moderate or large shock.—Unless the seismic history is long, it is difficult to estimate the rate of occurrence of moderate or major shocks in such an area with high reliability. The lower Mississippi Valley is an example of this type of area. 4. Area of low seismicity with no known moderate shocks—Unless geologic information is available, no reliable method for estimating future seismicity is presently available for these areas. Special studies utilizing tempo- rary seismograph stations to investigate the occurrence of microearthquakes may provide important research information, but the rela- tion between the level of microearthquake activity and the occurrence of large (or even intermediate) earthquakes is not clear. The Atlantic and Gulf Coastal Plains geologic provinces are examples of this type of area. Probably the most useful data for estimating the upper-bound magnitude in a region and recurrence rates of moderate to large earthquakes are the lengths of rupture in recent and prehistoric faulting. Length of rupture seems to be closely related to the magnitude of the earthquake (Wallace, 1970; Bonilla, 1970; Mark, 1977). Also, dating of the times of occurrence and the time intervals between fault ruptures is useful for es- tablishing recurrence rates. Present data are inadequate to resolve all the questions that have now arisen about the upper-bound magnitude for most re- gions of the United States. IDENTIFY SEISMOTECTONIC FEATURES The location and characteristics of faults and other tectonic structures in the region surrounding the site are essential information along with the historical seismicity for estimating the upper-bound magnitude and the locations of potential earthquakes. An analysis of faulting and tectonic activity in a region can be com- plex and time consuming, requiring data from three sources: (1) geologic data on faults and the ages of rock formations they displace, (2) seismological data on the areal distribution of seismicity in relation to known faults and tectonic structures, and (3) historical ac- counts which provide evidence that a fault has rup- tured at the surface or that earthquakes might reason- ably be associated with a particular fault. Faults that are active, such as the San Andreas and Hayward faults in California, usually exhibit clear geologic, seismological, and historical evidence of re- 10 PROCEDURES FOR ESTIMATING EARTHQUAKE GROUND MOTIONS cent and recurrent fault movement. In many cases, however, the information is not definitive and the fault tends to fall in the borderline category between active and inactive. Specific criteria (Clufi' and others, 1972) for recognizing an active fault are given in table 5. One rationale for classifying fault activity (table 6) uses the available geologic, seismological, and histori- cal data to specify four fault-activity classifications: ac- tive, potentially active, activity uncertain, and inact- ive. To quantify fault activity requires researching the scientific literature concerning specific faults, the in,- strumentally recorded seismicity of the region, and historical accounts of past earthquakes and their after- shock sequence. The dimensions of fault rupture can often be estimated from the distribution of aftershocks. Trenching may also be required. Data and maps may be obtained from many sources, including the United States Geological ' survey, state geological surveys, local universities, and private consulting firms. Faults are considered to be significant if they fall into one of the following categories: (1) faults crossing the site that are capable of rupturing during the life of the engineered structure, and (2) faults located either near the site or some distance from the site that are capable of generating large damaging earthquakes. Specification of faulting potential in the vicinity of the site is of particular importance, but subject to con— troversy. The fault is defined as a fracture along which differential slippage of adjacent earth materials has occurred. Surface faulting is differential ground dis- placement at or near the surface caused directly by TABLE 5.—Criteria for recognizing an active fault [from Cluff a'nd others (1972)] Data source Geologic Specific criteria Active fault indicated by young geomorphic features such as: fault scarps, triangular facets, fault rifts, fault slice ridges, shutter ridges, offset streams, en- closed depressions, fault valleys, fault troughs, sidehill ridges, fault saddles; ground features such as: ogen fissures, mole tracks and furrows, rejuve- nate streams, folding or warping of young deposits, ramps, ground-water barriers in recent alluvium, echelon faults in alluvium, and fault paths on young surfaces. Usuall a combination of these fea- tures is generated by ault movements at the sur- face. Erosional features are not indicative of active faults, but they may be associated with some active faults. Stratigraphic offset of Quaternary deposits by faulting is indicative of an active fault. Seismological Earth uakes and microearthquakes, when well 10- cate instrumentally, may indicate an active fault. Absence of known earthquakes, however, does not ensure that a fault is inactive. Historical manuscripts, news accounts, ersonal diaries, and books may describe past eart uakes, surface faulting, landsliding, fissuring or at er re- lated phenomena. Usually for a large earthquake, there will be several accounts in the historical rec- ord. Evidence of fault creep or geodetic movements may be indicated. Historical fault movement. Current criteria suggest that a fault . should be considered capable of permanent surface dis- placement if it is characterized by one or more of the. following: 1. Movement at or near the ground surface oc- curred at least once during the past 35,000 years or more than once during the past 500,000 years. 2. Instrumentally determined seismicity is di- rectly related to the fault. 3. A relation to another active fault exists such that movement on one active fault can be reasonably expected to be accompanied by movement on the other active fault. The controversy arises from the current lack of under- standing of the geologic processes that lead to the ac- cumulation of strain and the sudden fracture of rocks in the earth’s crust. Also, in seismically quiet regions like the Eastern United States, no seismic source zones can be identified with historic surface faulting. Cluff and others (1972), Krinitzsky (1975), American Nuclear Society (1975), and Slemmons and McKinney (1977) have published information and guidelines to aid in the evaluation of faults. The key factors in the assessment of a fault’s potential for producing an earthquake are: 1. fault length, 2. magnitude and nature of displacement, 3. geologic history of displacements, especially the age of latest movements, and 4. the relation of the fault to regional tectonic fea- tures. An example will illustrate the procedure for evaluat- ing fault activity. Figure 5 depicts the principal faults in the San Francisco Bay region in the vicinity of the city of Fremont, Calif. Because of their location, the Hayward, Mission, Silver Creek, and Chabot faults have the potential for producing significant ground motions at the Fremont site. The San Andreas fault, despite being a greater distance away, is also impor- tant because it is capable of producing very large earthquakes. Table 7 illustrates the evaluation. Housner (1970) developed an empirical relation for correlating fault rupture length and earthquake mag- nitude (fig. 6). The rupture lengths for the San An- dreas, Hayward, and Calaveras faults are plotted on the figure as a guide. The relation shows that (1) faults with rupture lengths of 32—40 km generally produce magnitude 5 to 7 earthquakes, (2) faults with rupture lengths of 40—113 km generally produce magnitude 7 to 7.5 earthquakes, and (3) faults with rupture lengths of 300 km or more generally produce earthquakes of magnitude 8 or greater. Exceptions to the general trend occur, for example, some magnitude-8 earth- IDENTIFY SEISMOTECTONIC FEATURES TABLE 6,—Classification of fault activity [From Cluff and others (1972)] 11 Potentially Uncertain Active active activity Inactive Historical __________ Surface faulting and No reliable report No historic activity. associated strong of historic surface earthquakes. faulting. Tectonic fault creep or geodetic indications of movement. Geolo ic __________ Generall oung deposits g have high displaced or cut by faultin . Fresh geomorphic features characteristic of active fault zones present along fault trace. Physical ground-water barriers in geologically young deposits. Geomorphic features characteristic of active fault zones subdued, eroded, and discontinuous. Faults are not known to cut or displace the most recent alluvial deposits, but ma be found in older a luvial deposits. Available information does not satisfy enou h criteria to estab ish fault activity. If the fault is near Geomorphic features characteristic of active fault zones are not present, and geologic evidence is avail- able to indicate that the fault has not moved in the recent past. Water barrier may be found in older materials. Geologic setting in the site, additional studies are necessary. Seismological ______ Earthquake epicenters are assigned to individual faults with a high degree of confidence. which the geomorphic relation to active or potentially active faults suggests similar levels of activity. Alinement of some earthquake epicenters along fault trace, but locations have a low degree of confidence. Not recognized as a source of earthquakes. TABLE 7.—Classification of selected faults relative to Fremont, Calif. [From Cluff and others. (1972)] Classification Eault of activity Criteria Hayward ______ Active Historical faulting. 0C. _ o _ Mission ______ Potentially active Geologic setting and micro- 3" — earthquake epicenters. \ Silver Creek __Tentatively active No geomorphic features, no \ ground water barrier, no \ earthquake epicenters. . PLEASANTON FAULI v; Chabot ________ Potentially active Geologic setting. 0 ”v3 ‘M'SS'ON FAULT Calaveras ____Active Geomo hic features, his- 0 torica faulting, and \ strong earthquakes. o | Tn Vlllv \‘\‘"AVWARD FAULT Pleasanton “HActive Documented tectonic fault ‘2‘ , \ creep. \ \suvsn CREEK \ AULT . . . . . 6: Bollnger ______ Potentially active Geologic setting, no ‘ a; near junction with characteristic geomor- 6( Calaveras. phic features. 0 " Tentatively inactive 37 " — away from Cala- D 10 20 30 4OKILOMETEHS \ l_l__;.l_l Hollister 123 FIGURE 5.—Principal faults in the vicinity (modified from Cluff and others, 1972). 122° of Fremont, Calif. veras junction. Parks ________ Potentially active San Andreas __Active Ground-water barrier in Tertiary-Quaternary gravels. Geomo hic features, his— toric aulting, and strong earthquakes. 12 1000 I_ I I I | I I I _ 1600 500 " ‘ 300 : SAN ANDREA: / _ / I 100 : I _ 150 I —52M I : m L=2.25xw e \/ — 3 50 _ HAYWAF i _ 80 I; :1 Lu 5 | | 7 E 2 — l .4 ;~ I i Z ’5 10 _ ' _ 16 ‘. Z : V | - I Lu _ CALAVERAS — l— _I f I — u I— 5 l 8 2 s 2 M I I I 5 < L=I82x10‘ — I— u. _ E I I l _ 5. / I I I E | ID : , I i I : 1.6 : / | I | — 0.5 ! I ! ' 0.8 3 / I I I I | | — | I l _ | | | 0.1 I I I I I I II I I 2 3 4 5 6 7 8 9 FIGURE 6.—Empirical relationship of fault rupture length and earthquake magnitude (modified from Housner, 1970). quakes have had fault rupture lengths that are less than 300 km long. Housner did not specify the statisti- cal distribution for his empirical relation. Mark (1977), however, showed that o- = 0.93 for the least squares fit to the magnitude and fault rupture length relation, and he argued that this relation is more physically meaningful than fault rupture length as a function of magnitude. SUMMARY OF UNITED STATES EARTHQUAKES The exact mechanisms that produce tectonic earth- quakes in various parts of the United States are still in doubt. The most extensively favored theory is that tec- tonic earthquakes are caused by slip along geologic faults. The correlation of earthquakes with fault slip is clear in the Western United States, but it is not very clear in the Central and Eastern United States. The seismicity patterns of the Western and Eastern United States are quite different, and the causative mecha- nisms are not well understood at the present time. A number of physical factors are different in the Eastern and Western United States and may be the cause, in part at least, of the difference in seismicity. The area east of the Rocky Mountains is generally characterized by low heat flow, a thick crust in which east-northeast compressive stress is typical, and PROCEDURES FOR ESTIMATING EARTHQUAKE GROUND MOTIONS Cenozoic epeirogenic uplift. The area west of the Rock- ies is characterized by high heat flow, a thin crust, and extensive late Cenozoic volcanism. The attenuation of seismic waves is also appreciably lower east of the Rocky Mountains than it is to the west (Nuttli, 1973a, b; Nuttli and Zollweg, 1974), suggesting a distinctly different crustal structure. The basic premise is that basement structures and crustal blocks are adjusting to epeirogenic forces and continued movements along old fracture lines developed at different times in the geologic past to produce the present seismicity. WESTERN UNITED STATES The complex tectonic pattern of eastern and south- central Alaska is created by the interaction of the Pacific and North American plates (Packer and others, 1975). Interaction between two crustal plates either creates, destroys, or preserves crustal material. Crust is created at oceanic ridges, destroyed at trenches and collision-type mountain belts, and preserved along transform and strike-slip faults. Because of the present plate interaction in Alaska, two types of boundaries occur (fig. 7). An active subduction zone, which extends 740° 0° 0° 3 0a \’\ No ,6: 700 Shumagin transition «6°C "Disturbed zone M E R I C A N FAIRWEATHER \ PACIFIC FAULT 60° P L A T E QUEEN CHARLOTTE ISLANDS FAULT p L A T E // 0 Juan da 0 . «6: o 500 KILOMETERS Fuca “"193 \U“ “II/ADA \ 500 |_J.._l_l_l_l \.. NITED ”TIES“ °1 9°“ . FIGURE 7.—Maj0r tectonic features along the Pacific-North Ameri- can plate boundary in Alaska (modified from Packer and others, 1975). IDENTIFY SEISMOTECTONIC FEATURES 13 from the Aleutian Islands to the vicinity of Mount McKinley, is defined by a line of volcanoes and a Be- nioff zone (Davies and Berg, 1973). In the east, a topo— graphic trench is defined in the Gulf of Alaska. The eastern boundary of the subduction zone is a series of faults that roughly define a transform fault system. This system is believed to be a westward ex- tension of the Queen Charlotte Islands fault (Tobin and Sykes, 1968). The exact characteristics of the faults that connect the Alaska subduction zone with the Queen Charlotte Islands transform fault are poorly de- fined and are still a major unsolved problem. The geology and seismicity of Alaska are complex and still not well known in many areas. For this rea- son, many hypotheses (for example, Plafker, 1965, 1967; Grantz, 1966; Gedney, 1970; Page, 1972; Van Wormer and others, 1974; Brogan and others, 1975) have been proposed to explain the origin of Alaska and its tectonic development and seismicity (fig. 8). At least 24 active faults have been identified in Alaska (Brogan and others, 1975). The dominant fault in the southen area is the Denali fault system (fig. 9). This system consists of relatively straight, right-slip segments that combine to create a 2,200-km-long arcuate fault that curves 30° and extends from Chatham Strait to Bristol Bay. The fault system is generally considered to be an extension of the Queen Charlotte Islands transform fault. Estimates of total right-lateral displacement on the Denali fault range from 80 to 250 km. Evaluation of the seismicity of California and Nevada shows that fault slips are the causative mech- anism (Allen and others, 1965; Albee and Smith, 1966; Ryall and others, 1966; Bonilla, 1967, 1970; Wallace, 1970; Allen, 1975). Seismicity data show that the earthquakes occur primarily at relatively shallow depths (S 16 km) in the crust. Tectonic surface ruptures related to historic (0—300 years before the present) and Holocene (0—10,000 years before the present) earth- quakes are common. The well-known San Andreas fault (fig. 10) is a transform fault zone, or system; it is the boundary be- tween the Pacific and North American plates. The Pacific plate carries a small piece of North America with it as it moves northward at a rate of about 3.2 cm/year along the right—lateral strike-slip San Andreas fault. 160°W 140° 120° 65°N 0 500 KILOMETEHS L__.|—J FIGURE 8.—Epicenters of earthquakes in Alaska between 1962 and 1969 (US. Dept. Commerce, 1970). 14 165° 160° 155° PROCEDURES FOR ESTIMATING EARTHQUAKE GROUND MOTIONS 150° 145° 140° I l f I r I 65° 50° \ .. h. r Alaska Peninsula '1 / ..... — / zr— ‘ \l " , ff; (934/ i . - / v. 550 a / v ' A]. ‘\ ,’~ " [Ll/Z— K/g‘uyflg f/ , _. r/ 7 o 100 200 300 KILOMETERS 1 / Kodiak Island 1 a l 1 . l l‘rTrT‘ 1‘ ITj’“ \ \ \\ u 7 A”: 2“ Coast Ranges I ANCHORAGE Kenai " } Peninsula FAULT ZONE EXPLANATION Observed fault Inferred fault Epicenter of 1964 Alaska earthquake . J I l | l I I 1 50° FIGURE 9.—Major faults in southern Alaska. A distinctive alinement of seismic activity occurs in southern California. This zone of activity branches from the main trend of the San Andreas fault zone in the vicinity of the Garlock fault (fig. 10) and trends northward into western Nevada, marking the eastern flank of the Sierrra Nevada. Garfunkel (1974) sug- gested that right-slip motion between the Pacific and North American plates on the San Andreas fault has produced a region of crustal extension in the Gulf of California and the Salton trough. When the Pacific plate encountered the Mojave block, rotation and internal deformation of the block and bending of the San Andreas system occurred. This resistance and bending was accommodated by distortion of the block and by left-slip on the Garlock fault and other west- trending faults as well as thrusting in the Transverse Ranges. 1 The characteristics of some of the California and Nevada fault systems are summarized below: 1. The San Andreas fault zone is the largest fault system in California. The right-lateral strike-slip system is approximately 1,000 km long and up to 80 km wide. Several de- structive earthquakes have occurred along the fault, for example, near San Francisco in 1836 and 1838 and near Fort Tejon in cen- tral California in 1857. The magnitude-8.3 San Francisco earthquake of April 18, 1906 caused a surface rupture of about 432 km. Creep occurs along sections of the fault (Na- son, 1973), but some sections of the fault in both northern and southern California ap- pear to be locked at the present time and may be the zones of future earthquakes. 2. The Hayward fault was the source of two large earthquakes in 1836 and 1868. The 1836 earthquake caused a surface rupture of 64 \ IDENTIFY SEISMOTECTONIC FEATURES 124° 120° 116 ‘\ I l l l I I O R E G o N [/ — \ _ _ I D A H O \ \ _ _ \ . r — \ _ _ K I - - \ _ _ ' s- — _. / l I, 40° I h / I t I l PLEASANT VALLEY I ‘— \ FAULT SCARP ‘\ 19502 RAINBOW MOUNTAIN “"5 ‘ STILLWATER EARTH ‘ QUAKE FAULTING DIXIE VALLEv ,' l FAULT SCARP ‘ REI‘IIO. August 23 1954 December 16,1954 \ ) Ju/y1954 1903”, NEVADA :UTAH D mber 76, 1954 k“ C A L I F O R N | A FAIRVIEw PEAK “‘9 FAULT SCARP \ CEDAR MOUNTAIN \ 7932 \\ HAYWARD . SACRAMENTO EARTHQUAKE FAULT \FAULTING E h 1836 7934*ExcELsIOR MOUNTAIN | 1906 o3, CALAVERAS FAULT FAULTING I SAN FRANCISCO a}. 1868 masl ‘2‘}, \\ l \ \ \ ‘ _ -~| 1872 \ I l \ \\ \ £71, \ SIERRA NEVADA \ 1934 FAULT LAS VEGAS \ \7966 g I \ ‘V‘ 1922 \ I \\; ‘96“ FAULT I 7952 \GP‘“ I \ \ \ l \ I I ,94, A R I Z O N A K ' \ \ \ \-_ ‘2, \ \ \ ‘\\ \ch \ \ \ \\ p \ \ 34° __ 40 9'91 4 \\ r971“0\ (/47. \\ LOS ANGELES‘ \ EXPLANATION — — Recently active fault W Surface rupture m Two recorded ruptures SAN DIEGO 50 100 ture of 32 km 150 KILOMETERS km and the 1868 earthquake caused a rup— FIGURE 10 —MaJor faults 1n Callfomla and Nevada and locatlons of past surface ruptures 3 The Calaveras fault system w1th a length of approx1mately 160 km 1s one of the largest 1n northern Callfornia. It is prlmarlly a rlght lateral strike-slip fault w1th a vertlcal 15 16 10. 11. 12. PROCEDURES FOR ESTIMATING EARTHQUAKE GROUND MOTIONS component responsible for upward move— ment on the west side. The vertical compo- nent is estimated to be about 90.4 m. Creep presently occurrs along the fault near Hol- lister (Rogers and Nason, 1971; 'Mayer- Rosa, 1973). . The 240-km-long Garlock fault in southern California has a left-lateral movement and separates the Mojave Desert province from the Sierra Nevada and Basin and Range provinces. Only small historic earthquakes have been attributed to this fault system. The Sierra Nevada fault zone was the source of the 1872 Owens Valley earthquake which had an estimated magnitude of 8.3. Many people consider this earthquake to be the largest in California. The White Wolf fault zone was the source of the 1952 magnitude-7.7 Kern County earthquake. The fault motion was mainly dip-slip and produced a surface rupture of about 52 km. . The Imperial fault, a southern extension of the San Andreas fault system, was the source of the magnitude-7.0 1940 Imperial Valley earthquake. The surface rupture was about 64 km. . The 1971 San Fernando earthquake caused a 19-km surface rupture on a segment of the Sierra Madre fault system. The epicenter was about 4.8 km north of the San Gabriel fault system. Faulting was associated with the Transverse Ranges structural province, a region noted for its relatively young tec- tonic deformation, and it is the first example of surface faulting within that province. . The San Jacinto fault zone is one of the most seismically active in California (Thatcher and others, 1975). Thirteen large earth- quakes have occurred since 1890 along the 240-km-long fault. The 1899 earthquake south of Hemet, Calif, ruptured the ground surface for 19 km. The Pleasant Valley fault scarp in Nevada was associated with a magnitude-7.6 earth- quake in 1915. The surface rupture along the normal fault system was 32—64 km. The Dixie Valley fault scarp in Nevada is an example of a normal fault system. It was associated with the 1954 magnitude-6.8 earthquake which produced 61 km of sur- face rupture. The magnitude-7.3 Cedar Mountain, Nevada, earthquake of 1932 produced surface rup- ture of approximately 61 km along the nor- mal fault system. 13. The magnitude-6.5 Excelsior Mountain, Nevada, earthquake of 1934 produced 1.5 km of surface rupture. Ryall (1977) pointed out that active faulting in California is confined fairly well to single belts such as the San Andreas fault zone, but it is fairly evenly dis- tributed over the entire western Basin and Range prov- ince in Nevada. He argued that neither the historic seismicity nor the distribution of active faults is by itself sufficient to determine the earthquake potential in Nevada and that the seismic cycle in Nevada corres- ponding to the rerupture time of major faults is of the order of several thousands of years. The Puget Sound area of Washington is another zone of seismic activity along the Pacific Coast. The earth- quake—related fault breaks have not been clearly iden- tified in the area and the exact cause of seismicity has not been determined (Crossen, 1972; Rasmussen and others, 1975). According to Walper (1976), the seismic- ity may be related to a triple junction at the north end of the Juan de Fuca Ridge (fig. 7). Spreading motion is transmitted there to the Explorer Ridge which joins with another segment of the transform boundary re- gime represented by the Queen Charlotte Islands- Fairweather fault zone. The Puget Sound area has an anomalously thick crust compared to the rest of the Western United States (Warren and Healy, 1973) and may be an extension of Vancouver Island, which also has an anomalously thick crust (Berry, 1973). Thus, one hypothesis to explain the seismicity of the area is that the block of thick crust is a continental fragment caught up in a plate—boundary regime, moving inde- pendently to cause seismicity. The Intermountain Seismic Belt (Smith and Sbar 1974) is a zone of earthquake activity 1,300 km long and 100 km wide that extends southward from the south end of the Rocky Mountain front in western Montana, through Yellowstone, Wyoming, and eastern Idaho and then south-southwest through Utah into Arizona (see fig. 3, zone number 4). This seismic belt encompasses the Yellowstone mantle plume (Wilson, 1973) and is also coincident with the Cenozoic fault zones of the northern Rocky Mountains. Smith and Sbar (1974) interpreted the belt as a boundary between the northern rocky Mountain and the Great Basin sub- plates of the North American plate because it closely follows the boundary between major physiographic and structural provinces. The Wasatch fault (fig. 11) is the dominant fault in a system of generally north-south-trending active faults in the Intermountain Seismic Belt. It is a normal fault, down to the west, that extends for 370 km from Gunni- IDENTIFY SEISMOTECTONIC FEATURES 17 I 1 1 MaladCi 425 l D A H o W I I Ogden w Y o M I N G N SALTLAKECITY z ‘1 o o N Provo 40° — < I '5 > I 3 I P Lu 2 "‘ Nephi | 5 L; o "’ o ; Gunnison ,_ o ' 33 i I U l 0 38° —- _ __ U T A H __ __ ' ___ ' A R I z o N A I: l rn z x 0 so 100 KILOMETERS _ r" | n E o I I 114° 112° 110° FIGURE 11.——Wasatch fault zone. son, Utah, to Malad City, Idaho. More than 30 m of vertical displacement of late Pleistocene and Holocene materials has occurred in places along the Wasatch fault zone (Cluff and others, 1975). Only minor seismic- ity has been associated with the fault during the last 140 years. A 50- to 105-m-wide zone of damaged streets, curbs, houses, and buildings occurs in Salt Lake City along the fault trend. Approximately 85 per- cent of Utah’s population lives within 8 km of the Wasatch fault zone. The only historical earthquake in Utah that produced ground displacement was the 1934 magnitude-6.6 Hansel Valley earthquake. It occurred north of the Great Salt Lake. EASTERN UNITED STATES The earthquakes of the Mississippi Valley are asso- ciated with continuing tectonic activity at the upper end of the Mississippi embayment (fig. 12), a part of the Gulf Coastal Plain. The embayment is a structural trough that was formed in Paleozoic basement rock and subsequently filled with marine deposits. It is gen- erally thought that downwarping of the underlying Paleozoic basement rock is continuing today. Faults are thought to be present in the basement rocks in the embayment area but have not been conclusively iden- tified. With the possible exception of the 1811— 12 New Madrid earthquake, surface rupture has not been asso- ciated with historic or Holocene earthquakes in the Mississippi Valley area or any other area in the East- ern United States. Several fault systems have been identified in the Mississippi Valley area. The best-known faults in- clude: the New Madrid fault zone, the Kentucky River fault zone, the Mount Carmel fault, the Rough Creek fault zone, the Wabash fault zone, the Cottage Grove fault zone, the Saint Genevieve-Rattlesnake Ferry fault zone, and the Bowling Green fault zone. Some of these faults are shown in figure 10. None of these faults has exhibited either displacement or concentrated seismic activity in historical time. The New Madrid fault zone,.source of the 1811—1812 earthquakes and several thousand aftershocks, is pos- tulated to be a combination of normal fault zones trending northeastward from the northernmost extent of the Mississippi embayment to the east-west- trending Rough Creek fault zone. This zone has been inferred to exist beneath Cretaceous deposits in the embayment area. Fuller (1912) established his “epi- central line” on the basis of evidence of maximum ground surface disturbance from the 1811—1812 earth- quakes. Some investigators have suggested that the New Madrid fault zone can be traced across the Mississippi river flood plain in the vicinity of New Madrid, Mo.; others believe that the fault zone may extend as far north as Vincennes, Ind., or include the Wabash fault zone. The critical fact is that the caus- ative faults in the region of the Central United States where the largest historic earthquakes occurred are still not well identified today. The present seismicity in the New Madrid region has been determined by a regional seismic network (Stauder and others, 1976). The most significant find- ing is the identification of linear trends of seismic ac- tivity. These trends are 30—100 km long and are offset from or parallel to one another. The greatest number of earthquake foci lie very close to the surface of the Pre- cambrian rocks. Seismic activity is episodic, occurring in turn along some trends, then along others. When a particular linear segment is active, hypocenters are usually distributed along the whole segment. These phenomena suggest that the linear trends act as con- tinuous seismic features and are not dependent on forces acting at distant plate boundaries. Since 1963, one earthquake having body-wave magnitude (mb) greater than or equal to 4.75 has occurred about every 2 years. Kane and Hildenbrand (1977) reported on the good 18 PROCEDURES FOR ESTIMATING EARTHQUAKE GROUND MOTIONS ’s MISSISSIPPI if). / New Madrid ‘ ---\____ . .15- ___ If. ‘ A: :2" )0 1' v .. ‘1' POSSIBLE. ,'/ \ ) g V NEW MADRID ‘ r, _ N FAULT ZONE 6 . ) a, O '1 / ) ;@ m s _ \ \ (I) — _: MEMPHIS O 2 g”- '" 1: ”’ u: ’ l“ ‘3‘?" Q..’ .— 115} (4% ,1 ,, :3, Q‘ EXPLANATION O V ROUGH CREEK FAULT ZONE Maximum intensity and probable epi- center of earth- quake I City a . 35 0 Disturbance “a" Normal fault mwfiThrust fault O 50 100 150 KILOMETERS FIGURE 12.—Major tectonic features and historic earthquake activity in the Mississippi Valley area. correlation of the Mississippi embayment seismicity with a zone of subdued magnetic field having sharply defined parallel boundaries striking N. 45° E. The zone is about 100 km wide. The principal seismicity of the Mississippi embayment, including the 1811—1812 earthquake sequence, is located along the axis of the zone in a configuration similar to spreading ridges. The magnetic field data suggest that the zone is caused by a structural depression of 1—2 km relief below the Pre- cambrian basement, which lies at an average depth of 2—4 km. The general alinement of earthquakes and the axis of high seismic transmissibility extending from south- ern Illinois to the St. Lawrence Valley in Ontario and Quebec was noted by Woolard in 1958. He suggested that the alinement might be related to unloading of the Pleistocene ice cap. Sykes (1972) pointed out that sur— face faulting had not been observed for any of the earthquakes in the St. Lawrence Valley and that understanding of the tectonic mechanism was limited. In addition, Sbar and Sykes (1973) noted that large seismic gaps occur in the postulated seismicity trend and argued against the continuity of the southern Illinois-St. Lawrence trend. These questions are still unresolved. The earthquakes of the Appalachian region are asso- ciated with the Piedmont and the Appalachian Mountains, particularly the belt of thrust faults and folds. The seismicity pattern has been suggested to cor- relate with areas of high stress along preexisting fault zones that formed during continental collision (Ran- kin, 1975). Colton (1970) suggested that the 1886 Charleston, SC, earthquake was caused by movement on a late Precambrian or early Paleozoic aulacogen (a fault- bounded trough or graben) in the Carolina area of the Appalachians. The trends of historic epicenters (fig. 13) in this area (Bollinger, 1972, 1973) are not conclusive as to the mechanism. The seismic network in the Charleston area (Tarr and King, 1974) is providing evidence that the current seismicity is concentrated in discrete source zones, with the largest shocks being 7— 14 km deep. The fault-plane solutions suggest northeast-southwest compression. The mechanism for the New England earthquakes is controversial. No clear evidence of Holocene faulting is 35° 34 33 32° DETERMINE REGIONAL SEISMIC A’I'I‘ENUATION 1914,1924 Greenville 1953 1912, 1913 0 .1914, 1.965 / x 193 ’ £00195? I ,41799 1914 Florence 01945 I / 1.968 149/79,. Columbia 1 / o“ ’ P3“ Q\E°§(P.Cv\‘ I ,5 00 . OAiken 1945, 7.964 "929 Orangeburg _ EXPLANATION O Epicentral area of 1886 earthquake and its aftershock sequence 0 1914 ‘ Epicenter of other earthquake‘ and year of occurence area 0 40 80 KILOMETERS I I l I I 83° 82° 81° 30° 79° FIGURE 13.—Historic seismicity in South Carolina area (from Bol- linger, 1972). associated with the seismicity. Coney (1972) and Morgan (1973) suggested that the present seismicity is in response to the continuing adjustment of the crust to the horizontal drift of the North American plate and epeirogenic warping. They suggested that North America was over the Azores hot spot or plume before rifting took place to open the Atlantic Ocean. Sbar and Sykes (1973) suggested that the locations of earth- quakes in Eastern North America ‘are controlled by unhealed faults or fault zones in the presence of a high deviatoric stress and that the orientation of these faults with respect to the stress field may be a major factor in determining when an earthquake occurs in this area. There are presently many gaps in knowledge about the causative mechanisms for earthquakes in various regions of the United States. Progress in understand- ing has been slow. Expanded regional seismicity net- works and greatly improved knowledge of the 3-dimensional geologic structure over broad geograph- ic areas are needed before the earthquake potential in various regions throughout the United States can be specified precisely. DETERMINE REGIONAL SEISMIC ATTENUATION One of the most important factors in. specifying earthquake ground-motion precisely is knowledge of how seismic waves attenuate from the source in var- ious geographic regions of the United States. Research 19 on seismic attenuation has proceeded slowly because the physical parameters of the crust and upper mantle causing attenuation are difficult to quantify. Also, the present strong-ground-motion data are geographically limited, and few empirical data exist to define the ef-. fects of path parameters, such as: (1) the natural anisotropy and inhomogeneity of the earth; (2) loss mechanisms (geometrical spreading, absorption and scattering); (3) reflection, refraction, diffraction, and wave-mode conversion; and (4) wave interference. It will be a long time before adequate strong- ground-motion data are available for all parts of the United States; therefore, attenuation functions for areas outside California will have to be based on Modified Mercalli intensity data, which have deficien- cies. One significant deficiency of these relations is the lack of knowledge about the statistical distribution. A number of investigators (for example, Dutton, 1887; Lawson, 1908; Fuller, 1912; Gordon and others, 1970; Coulter and others, 1973; Evernden, 1975; Howell and Schultz, 1975; Trifunac and Brady, 1975c; Gupta and Nuttli, 1976; Brazee, 1976; Borcherdt and Gibbs, 1976; O’Brien and others, 1977; and Espinosaf 1977) have evaluated the intensity data of individual earthquakes or a set of earthquakes. These investigations have pro- duced results, some of which are now being applied in earthquake-resistant design. A detailed isoseismal map from a past earthquake is the best basis for deriving an intensity attenuation function for a region when ground-motion data are un- aVailable. The critical data contained on an isoseismal map are the values of maximum Modified Mercalli in- tensity reported at various locations. These values are transformed into an iso-intensity contour map. The contours can be deceiving, however, because isoseismal maps typically represent intensity values reported at sites underlain by alluvium or unconsolidated mate- rials. Because these sites generally undergo more in- tense ground motion than sites underlain by rock, at- tenuation functions derived from an isoseismal map, without regard for the local site geology, may overes- timate the ground motion level at the site of interest. Data showing the effect of site geology in central California on intensity increments are listed in table 8 TABLE 8.—Relatiue intensity values and ground character, central California [From Evemden and others (1973)] Ground character Relative intensity increments Granite __________________________________________________ —3 Jurassic to Eocene sedimentary rocks _________________ _-_—2% Oligocene, Miocene, and Pliocene sedimentary rocks __- ___—1’/2 Late Pliocene sedimentary rocks ____________________________ — 1 Quaternary sediments (not saturated) ______________________ —1/z Saturated or near-saturated alluvium or bay fill ____________ 21/2 20 PROCEDURES FOR ESTIMATING EARTHQUAKE GROUND MOTIONS (Evernden and others, 1973). The reports by Hopper and others (1975) and Rogers and others (1976) also contain information about the effects of the surficial geology on intensity for the Puget Sound, Wash. and Salt Lake City, Utah areas. Figure 14 shows the Modified Mercalli intensity isoseismal map for the 1971 San Fernando, Calif. earthquake. The area of very heavy ground shaking was 768 kmz. The total Los Angeles metropolitan area affected was about 1,536 km2 and a total populaton of about 7 million was exposed to the shaking. Isoseismal maps for the 1906 San Francisco and the 1811 New Madrid earthquakes are compared in figure 15. The very large difference in felt areas for these two earthquakes is characteristic of the difference in seis- mic wave attenuation in the Eastern and Western United States (Nuttli, 1971; 1973b). Evernden (1975) used the difference in seismic at- tenuation rates in the Eastern and Western United States as a basis for assessing the size of earthquakes. Using a predictive model for intensity that incorpo- rated regional variation in seismic attenuation, Evernden concluded that the 1906 San Francisco earthquake was more than 100 times larger than the 1811— 1812 New Madrid and 1886 Charleston earth- quakes. He suggested that the lengths of the fault breaks for the New Madrid and Charleston earth- quakes were on the order of 10—20 km, considerably smaller than had been assumed in the past. He also suggested that the 1872 Owens Valley, Calif. earth- quake was not so large as originally thought. Gupta and Nuttli (1976) derived the relation: NEVADA 35° San Diego 200 KILOMETEHS \\ 120° 115° 0 100 M LICO \"\ EX l '\.. FIGURE 14.—Isoseismal contours for 1971 San Fernando, Calif, earthquake. I(R)=Io+3.7—0.001R—2.7 log]? for R>20 km where I (R) is MM intensity at R, epicentral distance in kilometers. This relation is applicable for the Central United States in the area between the Rocky Mountains and the Appalachians. This empirical at- tenuation law is consistent with intensity data from past earthquakes in the Mississippi Valley and model studies (Herrmann and Nuttli, 1975a, b). A number of intensity attenuation relations have been proposed for use in the Eastern United States (for example, see Cornell and Merz, 1974; M&H Engineer- ing and Memphis State University, 1974; and Young, 1976). Two attenuation relations proposed for use re- spectively in the southern Appalachian seismic zone and in the central Mississippi Valley are shown in figure 16. Both sets of curves are for firm ground condi- tions. The most accurate procedure for defining the seismic attenuation function of an area is to use observed strong-motion accelerogram data to derive a family of empirical acceleration-attenuation curves. Such a fam- ily of mean acceleration attenuation curves (Schnabel and Seed, 1973) applicable for sites in the Western United States is shown in figure 17. Each site is as- sumed to be located on rock, material having a shear— wave velocity of at least 760 m/s at low (0.0001 percent) strain levels. These curves are somewhat controversial in terms of whether or not they underestimate the peak acceleration inside 20 km, but they are still used at the present time in many applications. The statistical dis- tribution for the Schnabel and Seed acceleration at- tenuation curves is unknown. Another set of peak ac- celeration attenuation curves proposed by Davenport (1972) is also shown in figure 17. These curves are based on regression analysis of the available strong- motion data sample and indicate substantially higher values of peak ground acceleration than the corres- ponding Schnabel and Seed attenuation curves. These empirical curves have not received wide use in the United States. Algermissen and Perkins (197 6) proposed that accel- eration attenuation in the Eastern United States can be estimated by modifying the Schnabel and Seed curves as shown in figure 18. These empirical curves are identical with the Schnabel and Seed curves in the distance range 45—50 km and conform to the Nuttli (1973a) curves at greater distances. The statistical dis— tribution is unknown for these proposed attenuation curves. Donovan (1973) used two sets of data to derive acceleration-attenuation relations. He used data from 515 worldwide earthquakes encompassing a wide range of magnitudes to derive a general relation (fig. 19). The geometrical standard deviation for the mean CHARACTERISTICS OF GROUND SHAKING 21 December 16,1811 April 18, 1906 M=8.3 M=7.5 500 KILOMETERS FIGURE 15.—Isoseismal contours for 1906 San Francisco and 1811 New Madrid earthquakes (modified from Nuttli, 1973b). ‘ curve was 2.01. When he analyzed the peak accelera- tion data obtained from the 1971 San Fernando earth- quake, he obtained a standard deviation of 1.62 (fig. 19). The distribution of data relative to the mean re- gression line was shown to be log-normal. Ground-motion data from California earthquakes, from nuclear explosions in Nevada, Colorada, and New Mexico, and from the aftershock sequence of the March 1975 Pocatello Valley, Idaho earthquake have been analyzed to define preliminary frequency-dependent seismic attenuation relations (Johnson, 1973; McGuire, 1977a; and King and Hays, 1977). Values of the distance attenuation exponent, B, were derived from pseudo relative velocity (PSR V) response spectra by least squares analysis (that is, PSRV=AR B where R is the epicentral distance and A and ,8 are constants derived in the least-squares analysis) and are listed in tables 9- 13 along with values of 0'. These data indicate that seismic attenuation rates are about the same in southern Nevada and California, and that high- frequency (5—20 Hz) seismic energy attenuates more rapidly in Colorado and northern Utah than in California and southern Nevada (fig. 20.) Values of the geometrical standard error of estimate range from 1.58 to 1.78 for the highly “calibrated” transmission path of southern Nevada (Lynch, 1973) and from 1.61 to 2.22 for the less well “calibrated” transmission path of Col- orado (Foote and others, 1970). They range from 2.26 to 2.73 when both distance and magnitude are consid- ered. DEFINE THE CHARACTERISTICS OF GROUND SHAKING EXPECTED AT THE SITE After specifying the location and magnitude (or epi- central intensity) of each potential earthquake and an appropriate regional attenuation relation, the charac- teristics of ground shaking expected at the site can be determined. To implement this step requires consid- eration of the earthquake mechanism and basic wave propagation theory although current design proce- dures are primarily based on empirical procedures. It is well known that the amplitude, temporal, and spectral characteristics of ground motion produced at a site by an earthquake are functions of the earthquake source mechanism, the epicentral distance, and the geometry and physical properties of the geologic struc- tures traversed by the body waves and surface waves as they propagate from the source to the site (fig. 21). Seismograms recorded at all distances are complex, but 22 PROCEDURES FOR ESTIMATING EARTHQUAKE GROUND MOTIONS / 1000 SOUTHERN APPALACHIAN SEISMIC: ZONE CENTRAL‘MISSIS'SIPPI :VALLEY: 800 800 5007 400 300 200 100 I I FIRM GROUND IIIII l FIRM GROUND llllll I 80 60 50 40 30 L/VV'» §\ \\ \\\\ DISTANCE FROM EPICENTER, IN KILOMETERS IIIll l ///// \ § \ 11111 I — “#010! “a I IIIIII N I IIIII IIIII [III] I L. IV V VI VII VIII IX X IV < MODIFIED MERCALLI INTENSITY VI VII VIII IX X XI FIGURE 16.—Empirical intensity attenuation curves proposed for the Southern Appalachian seismic zone and the central Mississippi Valley (after Young, 1976). BEDHOCK ACCELERATION, INg lllllll l 5 10 IIIIIIII Statistical distribution undefined DISTANCE FROM FAULT, IN-KILOMETERS PEAK GROUND ACCELERATION, lNg .9 d .9 O _‘ 0.001 1 IIIIIIII I IIIIIIII IIIIIII I I I I A = 0.279 90.80m R—1.64 a: 2.10 (R = focal distance. in kilometers) llllllll lllllllll lllllllll lllllIll 10 100 DISTANCE FROM EPICENTER, IN KILOMETERS FIGURE 17.—-Average value of peak acceleration A in relation to distance R from fault for earthquakes of various magnitudes M (proposed by Schnabel and Seed, 1973 (left) and Davenport, 1972 (right)). Dashed lines represent extrapolated values. 1000 CHARACTERISTICS OF GROUND SHAKING 23 1.0 I I l I | I I . . . . Modified Schnabel-Seed curves Schnabel-Seed curves §a E i C p < 5 ’4 a} ""9. ° ’e 2 0.01 — ' 0g, — 0 .001 — . _ 0.0005 ' 2 5 10 20 50 100 200 500 DISTANCE, VIN Kl LOMETERS FIGURE 18.—Schnabel and Seed acceleration-attenuation curves modified for use in the Eastern United States (from Algermissen and Perkins, 1976). they are especially complex in the near field (that is, source-to-source distances of a few fault rupture widths) where the seismic spectrum tends to be rather broad. In the near field, the ground motions are strongly influenced by the dynamics of the fault rup- ture, and site properties are less important than source properties in defining the characteristic features of the ground shaking. The source of an earthquake is believed to involve a fracturing process in which rock in an environment of shear stress fails. This process may be idealized in terms of an elasto-dynamic crack in an elastic region. The failed region extends rapidly at its periphery to become the “fault surface.” The loss of cohesion across the region of failure translates into a boundary condi- tion in which the faces of the crack are free of shear stress. Assuming that the faces of the crack do not move apart, the rupture grows in a shear mode. A key to a physical understanding of the earthquake source problem, therefore, lies in understanding the physical processes that take place at the tip of the propagating crack. Body (P, SH, and SV) and surface (Love and Rayleigh) seismic waves are generated by the complex physical processes which occur during and after the rupture along the fault surface. The body waves are characterized by high frequencies (2—10 Hz) and com- monly produce the peak ground acceleration on the ac- celerogram. Rayleigh and Love waves travel and at- tenuate more slowly than body waves and have lower fundamental frequencies of vibration (for example, <1 Hz). The physics of the earthquake source is still not completely understood although a great deal of re- search is being concentrated on this problem. Brune (1970) and other scientists have shown on the basis of earthquake fault models that the earthquake source parameters, rupture velocity, effective stress, seismic moment, fault length, and stress drop, appear to have a predictable effect on the radiated seismic signal. In the far field (that is, source-to—station distances greater than about ten fault rupture widths), the high- frequency characteristics of the broad-band spectrum of the seismic signal are determined primarily by the dynamic stress drop. The low-frequency characteristics are determined by the seismic moment. The character of the radiated seismic signal is strongly dependent on the rupture velocity, which in turn is directly related to the effective stress available to accelerate the fault mo- tion. In the near field, the effective stress primarily determines the high-frequency wave characteristics, and the permanent static displacement determines the low-frequency wave characteristics although wave scattering and attenuation can affect the spectral com- position of the seismic signal at any distance. None of the source parameters, stress drop, seismic moment, and rupture velocity are used in current earthquake- resistant design. The source parameters, focal depth, rupture velocity, elastic constants of the source medium, stress drop, the fault configuration at depth, and source multiplicity, are probably the ones whose influence on ground shak- ing is least understood at present. The effect of focal depth may be of practical importance for nuclear power plant siting and other applications because the spectral composition of strong ground motion is strongly influ- enced by energy partition between body and surface waves, which is in turn influenced by focal depth. The effects of the differences in focal depth in California (S 16 km) and in the Puget Sound area (50—60 km) on ground motion are not clear. Ground motion can be described in a wide variety of ways (for example: as a time history, as spectra, and as peak ground-motion parameters). Almost all these de- scriptive formats are used in at least one of the various applications that require characterization of earth- quake ground motions. These formats will be described below. THE SEISMOGRAM The seismogram, whether written by a broad-band strong motion accelerograph system or on the short- and long-period seismographs of WWSSN (World-Wide PROCEDURES FOR ESTIMATING EARTHQUAKE GROUND MOTIONS 10 IIIII I 24 1000_1HH, “' "I'll” I I IIIIIII _ : \\T A 21320 90'58 M (R+25)_'|52 : — ' MEAN+ \ = _ E~\ . STANDARD \ 0' 2.01 _ :\8) because the rupture dimensions of the earthquake exceed the wavelengths 28 1.5 I I g, 10 _ S. 16°E. component, _ E ' ground acceleration g 0.5 E c: 0 “J _l B -o.5 D < I .- D l | S. 160E. component ground velocity ‘ m C VELOCITY, IN § 1 IN CENTIMETERS CENTIMETERS PER SECOND 50 I E S. 16" E. component, % ground displacement 2 0 .J O. ‘2 o -5o l | l] 5 10 15 TIME, IN SECONDS FIGURE 23.—-S. 16° E. accelerogram recorded at Pacoima Dam and the velocity and displacement seismograms derived from it; 1971 San Fernando, Calif. earthquake. of the seismic waves used in the magnitude determina- tion. Although seismic moment is an interpretative quantity, its greatest advantage is that it can be inter- preted simply in terms of the source mechanism; that is, the moment is proportional to the product of the fault area and the displacement across the fault. The magnitudes and seismic moments derived for a number of southern California earthquakes are listed in table 14. Although additional research is needed to establish all the physical facts about seismic moment and its applicability in earthquake-resistant design, one of the most interesting results obtained to date is the remarkable linearity that exists between the log- arithm of the seismic moment and the logarithm of the fault area (Thatcher and Hanks, 1973). This linearity suggests a constant average stress drop of about 10 bars in earthquakes. PEAK GROUND ACCELERATION Current practice in design of earthquake-resistant structures is to use peak ground acceleration as a measure of the severity of ground motion even though peak acceleration may not be the best parameter to represent this characteristic of ground motion. Peak acceleration is used because of its familiarity and wide acceptance in the engineering community as a meas- PROCEDURES FOR ESTIMATING EARTHQUAKE GROUND MOTIONS Trace amplitude = 23 mm I I | | I I rIIIIINIfMIIIII' I 1 2 | I I /-----S-Ptime=24s I / I I l 500 I l 1—-5II | I 400— I I —40 -l "m , \ 6‘ / \ 300— _ —5o/ \\ —30 l \ 5 —20 200 —zo - _,0 4.. _ "’5 mu _m _ 60—_6 3— b2 40— - _..4 "1 2— -—0.5 20- 1- —o.2 —2 0-5- —o.I 0— AMPLITUDE (mm) MAGNITUDE SCALE DISTANCE (km) I S-P TIME (s) FIGURE 24.—Determination of Richter magnitude. Using the maximum amplitude of the seismogram and the difference in arrival times of the P and S waves, the value of magnitude can be read from the nomogram. ure of the lateral forces on high-frequency structural systems. For intermediate- and low-frequency systems, ground velocity and displacement data are more appli- cable. Values of peak horizontal ground acceleration, tabu- lated in table 15, have been used by various inves- tigators to develop empirical relations for estimating horizontal ground motions. TABLE 14.—Magnitude and seismic moments of southern California earthquakes [From Hanks (1976)] Geographic Seismic moment Date locality Magnitude (10’5 dyne-cm) Jan. 9, 18571 ______ Fort Tejon 8 + 900 March 26, 1872' -_ Owens Valley 8 + 500 July 22, 18991 ____ San Jacinto 7 i 15 March 11, 1933 ,-Long Beach 6.3 2 May 19, 1940 -11- Imperial Valley 7.1 30 July 21, 1952 W, Kern County 7.7 200 April 9, 1968 ____ Borrego Mountain 6.4 6 Sept. 12, 1970 W" Lytle Creek 5.4 .1 Feb. 9, 1971 ______ San Fernando 6.4 10 'Noninstrumental magnitude assignment. CHARACTERISTICS OF GROUND SHAKING 29 TABLE 15.—Values of peak horizontal ground acceleration recorded in past earthquakes and used for estimating horizontal ground motions in earthquake-resistant design ‘ [From Seed, Murarka, Lysmer, and Idriss 1976] Approximate . source Maximum Soil distance Component Accleration depth Recording Earthquake Date Magnitude (km) of motion (g) (m) site Rock sites Helena, Mont ____________ 10/31/35 6.0 8 North—South 0.146 rock Carrol College, East—West .145 Helena. Kern County, Calif __________________ 7/21/52 7.7 43 North 21 East .156 do Taft, Calif. South 69 East .179 San Francisco, Calif __________________ 3/22/57 5.25 11 North 10 East .083 do Golden Gate Park, South 80 East .105 San Francisco. Lytle Creek, Calif __________________ 9/12/70 5.4 13 South 25 West .197 do Wrightwood, Calif. South 69 East .142 ________ Do. _______- 19 North—South .164 do Devils Canyon, East—West . 1 7 7 (slall' fBernardino, a 1 . Parkfield, Calif __________ 6/27/66 5.6 7 North 65 West .269 do Temblor, Calif. South 25 West .347 Borrego Mtn., Calif __________________ 4/8/68 6.5 134 North 33 East .041 do SCE Power Plant, North 57 West .046 San Onofre, Calif. San Fernando, ' Calif __________________ 2/9/71 6.6 3—5 South 16 East 1.24 do Pacoima Dam. North 76 West 1075 ________ Do. -___-___ 21 North 21 East .367 do Lake Hughes, North 69 West .287 Sta. 12. ________ Do. ----_-,- 24 North—South .164 do 3838 Lankershim East—West .147 Blvd;, Los Angeles. ________ Do. ________ 26 South 69 East .188 do Lake Hughes, South 21 West .394 Sta. 4. ________ Do. ________ 30 South 08 East .217 do Santa Felicia Dam South 82 West .202 (outlet) ________ Do. ________ 31 North—South .180 do Griffith Park East—West .171 Observatory. ________ Do. ________ 30 North—South .089 do Cal. Tech. Selsmol. East—West .192 Lab., Pasadena. ________ Do. ________ 40 North 03 West .176 do Santa Anita Dam. North 87 East .213 Stiff soil sites Lower California ________ 12/30/34 6.5 58 North—South 0.160 32 El Centro, Calif. East—West .182 Im erial Valley, alif __________________ 5/18/40 7.0 8 North—South .33 32 Do. East—West .21 San Francisco, Calif __________________ 3/22/57 5.25 16 North 09 West .043 45 Alexander Bldg., North 81 East .046 San Francisco. ________ Do. ________ 17 South 09 East .085 64 State Bldg., South 81 West .056 San Francisco. Parkfield, Calif __________ 6/27/66 5.6 .1 North 65 East .489 48 Cholame-Shandon 2. ________ Do. ________ 5 North 05 West .354 32 Cholame-Shandon 5. San Fernando, Calif __________________ 2/9/71 6.6 21 North 21 East .315 19 Castaic, Old Ridge North 69 West .270 Route. ________ Do. ________ 35 North—South .170 64 Hollywood Storage, East—West .211 RE. Lot, Los Angeles. ________ Do. ________ 39 North—South .136 14 3470 Wilshire Blvd. Los An eles. ________ Do. ________ ‘ 39 North 9 West .153 32 3550 Wils ire Blvd., Los Angeles. ________ Do. ________ 28 North 11 East .225 22 15250 Ventura Blvd., North 79 West .149 Los Angeles. ________ Do. ________ 28 South 12 West .243 22 14724 Ventura Blvd., Los An eles. ________ Do. ________ 39 North—South .161 13 3407 W. ixth St., East—West .165 Los Angeles. 30 TABLE 15.—Values of peak horizontal ground acceleration recorded in past eart earthquake-resistant design— ontinued PROCEDURES FOR ESTIMATING EARTHQUAKE GROUND MOTIONS uakes and used for estimating horizontal ground motion in Approximate source Maximum Soil distance Component Accleration depth Recording Earthquake Date Magnitude (km) of motion ) (m) site Deep cohesionless soil sites Western Washington ____________ 4/13/49 7.1 20 South 04 East 0.165 132 Highway Test Lab., South 86 West .280 Olympia, Wash. Kern County, Calif __________________ 7/21/52 7.7 127 North—South .047 1 12 Cal. Tech., East—West .053 Athenaeum, Pasadena. Eureka, Calif ____________ 12/21/54 6.5 25 North 11 West .168 80 Federal Bldg, North 79 East .257 Eureka. ________ Do. ________ 30 North 44 East .159 160 City Hall, North 46 West .201 Ferndale. Puget Sound, Wash __________________ 4/29/65 6.5 58 South 04 East .137 134 Highway Test Lab., South 86 West .198 Olympia, Wash. Ferndale, Calif __________ 12/10/67 5.6 25 North 46 West .105 160 Cit Hall, South 44 West .237 Flerndale. San Fernando, Calif ________________ 2/9/71 6.6 16 North—South .255 176 8244 Orion Blvd., East—West .134 Los Angeles. ________ Do. ________ 19 North—South .117 176 15107 Vanowen St., East—West .1 10 Los Angeles. ________ Do. ________ 37 North—South .095 112 Cal. Tech., East—West .109 Athenaeum, Pasadena. ________ Do. ________ 37 North—South .202 112 Cal. Tech, East—West .185 Millikan Library, Pasadena. ________ Do. ____--__ 30 North—South .141 148 Cal. Tech., Jet East—West .212 Propulsion Lab., Pasadena. Sites with soft-to-medium clay and sand San Francisco, Calif __________________ 3/22/57 5.25 18 North 45 East 0.047 91 So. Pacific Bldg, North 45 West .046 San Francisco. Alaska __________________ 4/3/64 6.0 131 North 13 East .044 110 Elmendorf AFB, North 77 West .051 Anchorage. In current design practice, it is common to assume that the maximum ground accelerations in the two horizontal directions are equal and that the maximum vertical acceleration is two-thirds or more of the maximum horizontal acceleration. The assumption of equality for the two horizontal components of ground motion is not always correct, so alternate approaches such as the spectrally maximized record (Shoja-Taheri and Bolt, 1977) may be needed in some design applica- tions. The assumed ratio of 2/3 for peak vertical to peak horizontal ground acceleration has a probability of ex- ceedance of 23 percent (Werner and Ts’ao, 1975). A number of investigators, (for example, Housner, 1965; Davenport, 1972; Boore and Page, 1972; Page and others, 1972; Schnabel and Seed, 1973; Dietrich, 1973; Orphal and Lahoud, 1974; Trifunac and Brady, 1975a; Seed, Murarka, Lysmer, and Idriss, 1976; Trifunac, 1976a; Hanks and Johnson, 1976; and Boore and others, 1978) have studied various aspects of peak ground acceleration. Some of these studies and their conclusions are summarized below: 1. Hanks and Johnson (1976) studied a set of strong-motion accelerograms and showed that the causative processes that generate peak ground accelerations within approxi- mately 10 km of the source are independent of magnitude (M) for 4.5SMS7 .1. This result is important because current design practice assumes that peak acceleration is strongly de- pendent on magnitude. 2. Trifunac and Brady (1975a), on the basis of re- gression analysis of the strong-motion data recorded in the Western United States from 1933 to 1971, suggested that peak accelera- tions at the fault, for the frequency band 0.07—25 Hz, probably do not exceed about 3—5 g. They proposed that the maximum acceler- ation may be reached at a magnitude of 6.5— 7.0. Both conclusions are controversial, main- ly because of limited data and overemphasis of data from the San Fernando earthquake. 3. Seed, Murarka, Lysmer, and Idriss (197 6) noted that peak amplitudes of ground acceleration .measured at sites on rock in the Western CHARACTERISTICS OF GROUND SHAKING 31 United States exhibit very little difference statistically from peak amplitudes measured at sites underlain by less than 48 m of stiff clay, sand, or gravel. At sites where rock is overlain by at least 80 m of cohensionless soil, the mean recorded peak acceleration is greater by factors of 3.5—4.0 than at rock sites for weak ground motions on the order of 0.003 g. The effect is reversed for peak accelera- tions at rock sites between 0.1 and 0.7 g, and these peak accelerations tend to exceed those recorded at deep cohesionless soil sites by as much as 80 percent. 4. Dietrich (1973), using finite element techniques to model various earthquake sources, showed that peak ground acceleration is proportional to stress drop. Hence, the variability in peak ground acceleration is a function of the var- iability in stress drop. Data to quantify the variability of stress drop are scarce. Rogers, Perkins, and McKeown (1976) estimated on the basis of the available data that stress drop has a log-normal distribution with a mean stress drop of about 20 bars and a geometrical standard deviation of 3.7. The range of horizontal peak acceleration for sites on rock in the Western United States is shown in figure 25. These curves are based on limited data, especially for source-to-site distances of less than 20 km, which causes disagreement about whether the “probable 0.9 0.8 ’7‘ Oroville, Calif. s. 07 K earthquake (ML=4'7) z . e - ;. \\ , Stone Canyon, Calif. .53 0.6 ,4 ( earth'quake (ll/I\L=4.7i) E i \‘(Probable upper bound d 0.5 X \ W)» W x E 0.4 / 4 Ml/7\6\\ cc 9 . \ s 0, % a>. \ = - V ///x \ E «QM-— 5.6 Vme \ g 02 / / bx \\ 7 -2 W?» > \ ”a. \ 0.1 {#4 I / 77x ‘ \\ 0 3.2 6.4 9.6 16 32 64 96 160 DISTANCE FROM CAUSATIVE FAULT, IN KILDMETERS FIGURE 25.—Range of horizontal peak acceleration as a function of distance and magnitude for rock sites in the Western United States (from Schnabel and Seed, 1973). upper bound” is correct. For example, the 1975 Oroville, California earthquake (ML=4.7) produced a peak ground acceleration of 0.7 g at a distance of 11.2 km from the fault. The 1972 Stone Canyon, California earthquake (M L=4.7) produced peak ground accelera- tions of 0.19, 0.63, and 0.16g at distances of 3, 6, and 7 km, respectively. The 1971 San Fernando, California earthquake produced a peak acceleration of 1.2 g at Pacoima dam and also exceeded the bounds proposed by Schnabel and Seed (1973). The explanation for these observations is that they are “rare” points lying at the extremes of the distribution of the acceleration data sample or that the curves proposed by Schnabel and Seed depict peak ground accelerations that are too low, especially those close to the fault. PEAK GROUND VELOCITY AND DISPLACEMENT Ground motion can also be characterized by velocity and displacement time histories derived from an ac- celerogram and their peak values. Ground velocity and displacement values (table 16) seem to have a more determinant upper bound than ground acceleration. Displacement time histories and spectra are used in defining seismic moment, stress drop, and source di- mensions and are also used in wave propagation studies. The amplitudes of ground displacement are propagated more coherently than amplitudes of ground velocity and acceleration because their low-frequency spectral composition is not very sensitive to scattering by small geologic inhomogeneities. Velocity and dis- placement time histories are needed, respectively, for modeling earthquake effects on intermediate- and low-frequency structural systems. Velocity time his- tories can be used in canjunction with the shear-wave velocity of the surficial materials to estimate the level of shear strain induced in the soil and rock through the relation 01 = 1 flu 0x B at where (iii is the shear strain level, 6_u is the peak 6x . 6x ground velocity, and ,8 is the shear-wave velocity of the material. 7 Various investigators (for example, Newmark and Rosenbleuth, 1971; Page and others, 1972; Ambraseys, 1973; Trifunac and Brady, 1975a; Trifunac, 1976a; Seed, Murarka, Lysmer, and Idriss, 1976; and Boore and others, 1978) have evaluated peak velocity and displacement in terms of distance and magnitude. Some of the important conclusions of these studies are: 1. Trifunac and Brady (1975a) suggested that, at the 90-percent confidence level, the peak ground velocities and displacements at the 32 PROCEDURES FOR ESTIMATING EARTHQUAKE GROUND MOTIONS TABLE 16.—Values of peak horizontal ground velocity and displacement derived from accelerograms of past earthquakes and used for estimating horizontal ground motions in earthquake-resistant design [From Seed, Ugas, and Lysmer (1976)] Approx- imate source Maximum Maximum distance Component velocity displace- Recording Earthquake Date Mamitude (km) of motion (cm/s) ment (cm) site Rock sites Helena, Mont __________ 10/31/35 6.0 8 North—South 7.3 1.4 Carroll College, East—West 13.3 3.7 Helena. Kern County, Calif __________________ 7/21/52 7.7 43 North 21 East 15.7 6.7 Taft, Calif. South 69 East 17.7 9.2 San Francisco, Calif __________________ 3/22/57 5.25 11 North 10 East 4.9 2.3 Golden Gate Park, South 80 East 4.6 .8 San Francisco. Lytle Creek, Calif ________________ 9/12/70 5.4 13 South 25 West 9.6 1.03 Wrightwood, , South 69 East 8.9 2.21 Calif. ...... Do. __-_-_ 19 North—South --____ ______ Devils Canyon, East—West ____________ San Bernardino, Calif. Parkfield, Calif ________________ 6/27/66 5.6 7 North 65 West 14.5 4.7 Temblor, Calif. South 25 West 22.5 5.5 Borrego Mountain, Calif ________________ 4/8/68 6.5 134 North 33 East 3.7 1.6 SCE Power Plant, North 57 West 4.2 2.9 San Onofre, Calif. San Fernando, Calif __________________ 2/9/71 6.6 3—5 South 16 East 115.5 37.7 Pacoima Dam. North 76 West 57.7 10.8 ______ Do. __--__ 21 North 21 East 14.7 1.8 Lake Hughes, North 69 West 12.4 8.9 Sta. 12. ______ Do. __--__ ‘ 24 North—South 12.3 4.9 3838 Lankershim Blvd., East—West 15.0 5.4 Los An eles. ______ Do. _---__ 26 South 69 East 9.9 7.0 Lake Hug es, South 21 West 6.2 4.6 Sta. 4. ______ Do. ”A"- 30 South 08 East 9.9 7.0 Santa Felicia Dam, South 82 West 6.2 4.6 (outlet). ______ Do. ____-- 31 North—South 20.5 7.3 Griffith Park East—West 14.5 5.5 Observatory ______ Do. --____ 3O North—South 5.8 1.6 Cal. Tech. Seismol. East—West 11.6 5.0 Lab., Pasadena. ______ Do. __---_ 40 North 03 West 5.3 3.2 Santa Anita Dam. North 87 East 6.7 5.9 Stiff soil sites San Fernando, Calif ________________ 2/9/71 6.6 21 North 21 East 16.5 4.2 Castaic, Old North 69 West 27.2 9.3 Ridge Route. ______ Do. --____ 28 North 11 East 28.2 13.4 15250 Ventura Blvd., North 79 West 23.5 10.3 Los Angeles. ______ Do. ______ 28 South 12 West 31.5 18.3 14724 Ventura Blvd. North 78 West 17.8 9.5 Los Angeles. ______ Do. ---___ 35 North—South 16.5 8.0 Holl ood Storage, East-West 21.1 14.7 P. . Lot, Los Angeles. ______ Do. --____ 39 North—South 22.3 11.4 3470 Wilshire Blvd., East—West 18.5 11.6 Los Angeles. ______ Do. ______ 39 North—South 18.0 10.3 3710 Wilshire Blvd., East—West 22.1 12.9 Los An eles. ______ Do. -__-__ 39 North—South 18.3 9.0 3407 W. ixth St., East—West 16.5 10.3 Los Angeles. ______ Do. __---- 39 North—South 14.7 9.9 3345 Wilshire Blvd., East—West 16.1 9.1 Los Angeles. ______ Do. ______ 67 North 65 East 4.1 2.6 2516 Via Tejon. South 25 East 5.0 3.4 Sites underlain by deep cohesionless soil Puget Sound, Wash ________________ 4/29/65 6.5 58 South 04 East 8.0 2.7 Highway Test Lab., South 86 West 12.7 3.8 Olympia, Wash. San Fernando, Calif __________________ 2/9/71 6.6 16 North—South 30.0 14.9 8244 Orion Blvd., East—West 23.9 13.8 Los Angeles. ______ Do. ______ 19 North—South 31.5 17.5 15107 Vanowen St., East—West 28.8 15.3 Los Angeles. CHARACTERISTICS OF GROUND SHAKING 33 TABLE 16,—Values of peak horizontal round velocity and displacement derived from accelerograms of past earthquakes and used for estimating orizontal ground motions in earthquake-resistant design—Continued Approx— imate source Maximum Maximum distance Component velocity displace- Recording Earthquake Date Mamitude (km) of motion (cm/s) ment (cm) site ______ Do ______ 39 North—South 7.9 3.0 Cal. Tech., East—West 14.3 7.3 Athenaeum, Pasadena. ______ Do _-____ 37 North—South 9.8 2.7 Cal. Tech., East—West 16.3 6.9 Millikan Library, Pasadena. ______ Do ______ 30 North—South 9.0 2.9 Cal. Tech. Jet East—West 13.4 5.0 Propulsion Lab., Pasadena. fault for the frequency band 0.07—25 Hz probably do not exceed 400—700 cm/s and 200—400 cm. 2. Seed, Murarka, Lysmer, and Idriss (1976) pro- posed that the average maxima of strong mo- tion velocity and displacement may be reached for magnitude 6.5—7.0 earthquakes; however, the validity of their suggestion has yet to be proved. They proposed the attenua- tion relation shown in figure 26 for moderate earthquakes. DURATION Duration of shaking has been shown to be one of the most important parameters of ground motion for caus- ing damage. Some earthquakes (For example, Park- field, Calif, 1966, and Stone Canyon, Calif, 1972) have produced short, high-frequency accelerograms, but they have not caused structural damage, even though the peak ground accelerations were on the order of 0.5 g. In other earthquakes that produced damage, the peak ground acceleration level was less, but the dura- tion of shaking for a wide range of frequencies was greater. The 1966 Parkfield, Calif, and 1967 Koyna Dam, India, earthquakes both produced peak ground- accelerations and velocities as large or larger than those produced by the 1940 Imperial Valley, Calif. earthquake. The Imperial Valley earthquake caused more damage than either of the other two earthquakes although towns were about the same distance from each earthquake’s epicenter, mainly because the dura- tion of ground shaking was several times greater in the Imperial Valley earthquake. Duration of shaking also plays an important causa- tive role in liquefaction, a physical process that can also lead to damage. In the 1964 Alaska earthquake, for example, soil liquefaction developed about 90 sec- onds after the ground shaking began, but it has ’been postulated that it would not have occurred if the ground shaking had lasted only 45 seconds (Seed and Idriss, 1971). Since 1964, a number of investigators (for example, Esteva and Rosenblueth, 1964; Housner, 1965; Husid, 1967; Rogers, 1972; Page and others, 1972; Bolt, 1973; Hays and others, 1973; Perez, 1973; Housner, 1975; Hays, 1975a; Trifunac and Brady, 1975b; Dobry and others, 1977; Trifunac and Westermo, high-frequency accelerograms, but they have not caused structural damage, even though the peak ground accelerations were on the order of 0.5 g. In other earthquakes that produced damage, the peak ground 1977; and Hays and others, 1978) have studied duration of ground shaking. The studies by Trifunac and Brady (1975b) and Trifunac and Westermo (1977) were the most com- prehensive in scope, but all the studies have provided evidence that three interrelated parameters (1) the amplitude, (2) the length of time that the ground shakes, and (3) the dominant frequency at which it shakes, are the parameters that cause structural dam- age. Bracketed duration, a measure used frequently in the engineering community, is the time during which the acceleration level equals or exceeds some amplitude threshold such as 5 percent g. This parame- ter is illustrated in figure 27 for the accelerogram re- corded at Pacoima Dam during the 1971 San Fernando, Calif. earthquake. The general trend of increasing bracketed duration with increase in magnitude is also shown in figure 27. Trifunac and Brady (1975b) used a definition pro- posed by Husid (1967) to calculate the duration of strong earthquake ground motion. Their definition was based on 90 pecent of the integral of the squared time history, for example: T T T a (t)dt, v (t)dt, d (t)dt. 0 0 0 They correlated the integral (fig. 28) of the squared acceleration (a), velocity (v), and displacement (d) time histories with recording site conditions, magnitude, and epicentral distance and concluded: 1. The average duration for a "sof ” alluvium site is 5~6 seconds longer than for an inter- mediate site and 10— 12 seconds longer than for rock sites. 34 PROCEDURES FOR ESTIMATING EARTHQUAKE GROUND MOTIONS 200 100 10 PEAK VELOCITY, IN CENTIMETERS PER SECOND I 10 SITES A Rock O‘Stiff soil 0 Deep soil Cl Probable maximum vaIue suggested by Ambraseys (1973) 100 1000 DISTANCE FROM ZONE OF ENERGY RELEASE, IN KILOMETERS FIGURE 26.—Re1ation between peak horizontal ground velocity and distance from source of energy release for magnitude 6.5 earthquakes (from Seed, Murarka, Lysmer, and Idriss, 1976). 2. The average duration increases by 2 seconds for acceleration and about 5 seconds for dis- placement for each unit increase in earth- quake magnitude, 3. The average duration increases about 1—1.5 seconds for every 10 km increase in epicen- tral distance. , Trifunac and Westermo (1977) extended the proce- dure for defining duration. Instead of deriving the in- tegral of the squared seismogram, they calculated the integrals for 10— 15 squared band-pass filtered time histories of the original seismogram. Duration derived in terms of filtered time histories exhibits the same trends as stated above, but the variability is less. Frequency-dependent characteristics of ground motion (For example, amplification) can be identified in the band-pass filtering proceSs and eliminated from con- sideration in the analysis. SPECTRA Fourier techniques are the standard for spectral analysis of ground motion data. A Fourier spectrum resolves the ground-motion time history into an infin- ite series of simple harmonic functions in the frequency domain. The resulting transformation provides both an amplitude and a phase spectrum that are uniquely re- lated to the seismogram. Four amplitude spectra (fig.' 29) are used frequently in engineering seismology. Examples of uses include: (1) analysis of ground-motion data (Hudson, 1962; Hays and others, 1975a); (2) source-mechanism studies (Savage, 1966; Brune, 1970; Trifunac, 1972b); (3) seis- mic. attenuation studies (Hays, 1969; Trifunac, 1967b); (4) site amplification studies (Duke and Hradilek, 1973; Rorcherdt and Gibbs, 1976); and (5) soil- structure interaction (Liu and Fagel, 1973; Crouse and Jennings, 1975). The application proposed by Brune in CHARACTERISTICS OF GROUND SHAKING 35 "N a San Fernando earthquake, Pacoima Darn. fi) E_ 0.62 S.16°E. component 2 2 g.— g C Lu d g -0.62 5SECONDS I .a N 4: Bracketed duration measure “h u . ' D.uration20.059=12.25 FAULT RUPTURE LENGTH, IN KILOMETERS 5 10 20 30 70 140 260 500 5" i r r l l I 1 I ‘6’ g _ A Duration >0.05g _ g + Values proposed by Housner (1965) Z _. 40 — -——-Bracketed duration, D >0.l]5g ‘ E + ”3:" . — - -Bracketed duration, D >0.1y z ‘z‘ ,_ 30 -— - I LIJ .— E n: + ‘5 w E 20 .. APuget Sound, 1949 - E Aimperial Valley, 1940 a San Fernando, 1971 (Castaic) : A , - ’ ““““ “- ASan Fernando, 1971 (Paooima Dam) is + x 10 _ ’ - E ,’ APugel Sound, 1965 .— 5: / AKern County, 1952 - 3 / a I 0 i’KSqn Francisco, 1957- 1 I I i 0 5 5.5 '6 6.5 7 7.5 8 3.5 MAGNITUDE FIGURE 27.—Bracketed duration values for the S. 16° E. accelero- gram recorded at Pacoima Dam from the 1971 San Fernando earthquake (top; from Hays, 1975a), and bracketed duration as a function of magnitude and fault rupture length (bottom; modified from Bolt 1973). 1970 to deduce source parameters from the far-field displacement spectrum has stimulated much impor- tant research in seismology. The displacement spectrum, after being corrected for instrument re- sponse and path propagation, has a flat low-frequency level (fig. 30), which is used to define the stress drop and seismic moment. It is also has a corner frequency (the frequency where the high- and low-frequency trends interact) that is related to the radius r of the equivalent circular fault causing the earthquake. In engineering seismology the spectral characteris- tics of ground motion are normally displayed as re- sponse spectra, a form preferred by structural en- gineers for the study of building response. Structures respond as oscillating systems with fairly well defined periods of vibration, and their response to ground mo- tion is strongly dependent on its spectral composition and duration. Simple systems such as viscous-damped pendulum or mass-spring systems have been success- fully used to model structural elements (Blume and others, 1961). When such a model is excited by ground motion from an earthquake, it will respond by vibrat- ing. The vibratory motion is described by the well known differential equation 56+2wn hx+w,, 2x: —a(t) where x is the relative displacement between the mass and ground, "x anda'c' are the velocity and acceleration of the mass relative to the ground, h is the fraction of critical viscous damping, can is the undamped natural frequency of vibration of the system, and a(t) is the ground acceleration. The Fourier amplitude spectrum has a close relation to the undamped velocity response spectrum (Hudson, 1962). These two spectral respresentations, although fundamentally different, are essentially interchange- able in seismic data analyses. The response spectrum technique, proposed by Be- nioff (1934) and Biot (1943), is a method for determin- ing the maximum amplitudes of response of an ensem- ble of simple damped, harmonic oscillators (a narrow- band filter) when excited by a given ground-motion time history (fig. 31). The various response spectra are: (1) pseudo absolute acceleration (PSAA), (2) pseudo relative velocity (PSRV), (3) absolute acceleration (AA), (4) relative velocity (RV), and (5) relative dis- placement (RD). Each response characterization has a physical meaning. PSAA is a measure of the maximum elastic spring force per unit of mass. RD represents the maximum value of the relative displacement of the simple system during vibratory motion. PSRV gives an approximate index of the greatest velocity, relative to its base, of the center of mass of the resonant simple structure. PSRV can also be related to the maximum energy absorbed in the spring. For low damping, the PSRV spectrum provides an upper bound to the Fourier amplitude spectrum (Jenschke,1970). Response spectra for four values of damping (0, 2, 5, and 10 percent of critical) derived from the horizontal component accelerogram recorded at El Centro from 36 PROCEDURES FOR ESTIMATING EARTHQUAKE GROUND MOTIONS STATION UNDERLAIN BY ROCK STATION UNDERLAIN BY 100 M OF ALLUVIUM RADIAL 10— 1 RADIAL VELOCITY 0 VELOCITY _ :53 cm/s I I I I I l I I I I I I I II 5 IO 10!] _ TIME, IN SECONDS TIME, IN SECONDS .—- 11.2 cnI/s VELOCITY IN CENTI‘ METERS PER SECOND D I T O J" VZIIIdt IN PERCENT _I 0 ° W/l/ll/l/l/l/l/l/l/A I7// // // // ///////A DURATION=GSECONDS DURATION=IDSECONDS FIGURE 28,—Example of integral definition of duration of shaking for a site on rock and a site underlain by alluvium (from Hays and others, 1978i Ioouo I I I I I I I I I I I I I I I I I I I D Z O D g Iouo ——— __. a: If 03 ._ .- I I.“ [— I.” E g 103 -- __ LLI U E fig _ _ D m I— U a”: 1.0 —- __ (I) LLI D D t — - _I Q. E < 5 01 ——- __. a D 0 LI. 0.01 I I I I I I I I I I I I I I I I I I I 031 04 In 103 Iouo FREQUENCY, IN HERTZ FIGURE 29.——Example of a Fourier amplitude spectrum derived from the accelerogram recorded at El Centre from the 1940 Imperial Valley, Calif., earthquake. the 1940 Imperial Valley, Calif., earthquake are shown velocity, acceleration, and displacement can be deter- in figure 32. Response spectra are typically plotted on mined simultaneously along with estimates of the peak tripartite log-log paper. From such a plot, the spectral ground acceleration and ground displacement. For CHARACTERISTICS OF GROUND SHAKING 37 BASIC RELATIONS a = corner frequency = 234$ , where ,3= shear-wave velocity and r is the radius of the circular source M0 = seismic moment =,u 1rr25, where 5 = average offset on fault and M = rigidity = gA a-r3 Aa- = stress drop = 27.4 [1,13 (lo/'3 Low-frequency trend (zero slope) \ \ — \ Slope of high-frequency I . . trend IS proportioned \to frequency '7 90 LOG SPECTRAL DISPLACEMENT p _.__.__._.____ LOG FREQUENCY FIGURE 30.—Schematic illustration of far-field displacement spectrum and some of the information about the source that can be derived from it. example, the peak value of horizontal ground accelera- tion is approximately equal to the value that the spectral acceleration approaches at very high frequen- cies (zero period), and the peak value of horizontal ground displacement is approximately the value which the spectral displacement approaches at very low fre- quencies (infinite period). Another spectral representation, the time-dependent response envelope (Trifunac, 1971; Perez, 1973; Hays and others, 1973), is shown in figure 33. The example shown is for the 1940 Imperial Valley earthquake ac- celerogram recorded at El Centro, Calif. It demon- strates the increased information content over the standard response spectrum. This spectral representa- tion is not widely used in earthquake-resistant design at the present time. Response spectra are widely used in engineering seismology research to define the frequency-dependent effects caused by parameters of the earthquake source and the transmission path. It is well known, for exam- ple, that the dominant spectral composition of ground motion shifts to the long-period end of the spectrum with an increase in earthquake energy release. Figure 34 illustrates the shift of corner frequency for two PSRV spectra representing, respectively, the San Fer- nando earthquake (M=6.6) and an aftershock (M=3.2). Both events were recorded at the same recording site (Glendale Municipal Building, Glendale, Calif.) and had essentially identical travel paths. The difference in spectral composition, therefore, primarily correlates with the difference in source parameters for the two events. It is also well known that the earth tends to act like a low-pass filter on propagating seismic waves. That is, the high-frequency spectral components are attenuated more rapidly than low-frequency compo- nents. This effect is shown schematically in figure 35 for response spectra derived from accelerograms rec- orded during the 1971 San Fernando earthquake. Frequency-dependent seismic-source scaling laws could be used in many design applications if they were available. Although the current practice of scaling ground-motion characteristics (for example, peak ac- celeration or response spectra values) in a linear man- ner to provide design ground-motion estimates is not satisfactory physically, the procedure has not been modified at the present time. Frequency-dependent distance-scaling laws that characterize the low-pass filtering effect of the earth in various geographic regions of the United States could also be used if they were available. Very few frequency—dependent attenuation relations have been developed because of the lack of ground-motion data. The few relations that are available (King and Hays, 1977) are preliminary estimates. Peak ground acceler- ation is attenuated to the site then used as the high frequency anchor for the site-independent response spectrum. For sites in the Eastern United States where only Modified Mercalli intensity data are available, in- tensity is attenuated to the site, converted to peak ac- celeration, and used to define the response spectrum. (Intensity and the design response spectra Will be dis- cussed in a later section.) A more accurate procedure for a scaling ground- motion data with distance could be used if data were available. Given a ground-motion response spectrum M 2 derived from a recorded accelerogram at epicentral distance R 2, the predicted spectrum M , of ground mo- tion at distance R1 is give by [3 M1:(&)M2 R2 where ,8 is the distance scaling exponent (see tables 9—13) for a particular period. A procedure such as this would accurately account for the known differences in regional attenuation. KNOWLEDGE GAINED FROM NUCLEAR EXPLOSION GROUND-MOTION STUDIES During the past decade, more than 300 underground nuclear detonations have been conducted at the Nevada Test Site mainly on Yucca Flat or Pahute 38 PROCEDURES FOR ESTIMATING EARTHQUAKE GROUND MOTIONS 100.0 I IIIIIII 5 percent damped I III!” I Illllll l Illlll 1 11111- 1 SPECTRAL VELOCITY, IN CENTIMETERS PER SECOND 8 O 1.0 l I l I III] 0.1 1.0 O UTPUT (RESPONSE TIME HISTORIES) U) 5 4o — I ' --— ‘. I — 40 I; _ FREQUENCY = 0.33 hertz. _ _ FRECUENCY = 2 hertz s I: I ‘ p g 20 — — _ — 20 2 U - ' _ _ _ 3 b“, 0 VM 1“ I" A A Av A A'MA'A A'.'A. A n E I: - V \ _ _ v v v _ I.“ E m 20 — — — — 20 U _ _ _ _ E 40 — — — 7 — 40 > a 5 III 15 20 25 o 5 10 15 TIME, IN SECONDS INPUT T0 RESPONSE SPECTRUM FILTER SYSTEM 2 (GROUND-MOTION TIME HISTORY) E 20 — — E I: S 10 — — .2. ° A A Lia 0 AM A- A 5‘5 v V V V E a III — __ U S 20 -— _ ”.1 > n 5 III 15 TIME, IN SECONDS FIGURE 31.-—The narrow-band-pass filtering involved in deriving a response spectrum. Mesa (fig. 36), located northwest of Las Vegas, Nev. These detonations ranged in yield from about 1 to 1,200 kilotons (Springer and Kinneman, 1971, 1975). More than 4,000 velocity and acceleration seismo- grams were recorded on paper and tape. These seismo- grams were recorded at 600 different recording sites, which were selected for either research purposes or safety documentation. The source-to—station distances ranged from 0.4 to 600 km. The stations are underlain by rock of various types and unconsolidated materials of various thicknesses. Eleven nuclear detonations have been conducted outside the Nevada Test Site. These detonations were as follows: Faultless and Shoal in central Nevada; Longshot, Milrow, and Cannikin on Amchitka Island, Alaska; Rulison and Rio Blanco in Colorado; Gnome and Gasbuggy in New Mexico; and Salmon and Ster- ling in Mississippi. Ground—motion data recorded from some of the im- portant nuclear detonations have been published (for example, Environmental Research Corporation, 1968, 1969, 1970a, b, c, 1974a; Hays and others, 1969; West, 1971; West and Christie, 1971). Comprehensive studies of the ground motion and structural response data have been performed and are summarized in reports by Environmental Research Corporation (1974b) and URS/John A. Blume, Engineers (1975). Research on nuclear explosion ground-motion data has established: CHARACTERISTICS OF GROUND SHAKING 39 M k0.08 T 1:10.18 0.08 0.18 M M T1 M M T005 TWO T01R50 LI"1(IIJ[I 05 10 50 100 FREQUENCY, IN HERTZ ' 0f single-degree-d-freedom systems I I | | I a o 0'1 6 N a m PSEUDO RELATIVE VELOCITY, IN INCHES PER SECOND S 5.1 N U1 4:- PSEUDO RELATIVE VELOCITY, IN CENTIMETERS PER SECOND 0.05 0.1 0.2 0.5 I 2 5 1O 20 50 FREQUENCY, IN HERTZ >7 Story I 3-7 Story I I-ZStorv Natural frequency range of building FIGURE 32.——Example of response spectra’derived from the 1940 Imperial Valley, Calif, earthquake accelerogram. M and k rep- resent the mass and spring constant of the equivalent mechan- ical system. Damping factor 0, 2, 5, and 10%. VELOCITY, IN CENTIMETERS PER SECOND 100.0 N0 RTH-SOUTH COMPON ENT PERIOD IN SECONDS TIME, IN SECONDS FIGURE 33.—Diagram showing time-dependent response envelope derived from the 1940 Imperial Valley, Calif, earthquake ac- celerogram. 1. The similarity of ground-motion response spectra of explosions and earthquakes (fig. 37) over the period range from 0.01 to 5 sec~ onds. 2. The approximate log-normal distribution of nu- clear explosion ground-motion spectral val- ues at a fixed period (Lynch, 1969, 1973). 3. The wide range (up to a factor of 10) in levels of ground motion that can occur because of dif— RADIAL COMPONENT 100.0 , , ., : SAN FERNANDO _\ EARTHOUAKE .. 2/9/71 M=6.6 11111 10.0 0.1 ESAN FERNANDO *_\ AFTERSHDCK _ 4/29/71 M = 3.2 PSEUDO RELATIVE VELOCITY, IN CENTIMETERS PER SECOND 0.001 0.01 0.1 1.0 10.0 PERIOD, IN SECONDS FIGURE 34,—Example of earthquake source effects on the spectral composition of ground motion. llIIIII I 100.0 PSEUDO RELATIVE VELOCITY, IN CENTIMETERS PER SECOND / ‘99 Isanlonlofrle‘t M l I I I/ J. 6 /4 0.1 . PERIOD, IN SECONDS FIGURE 35.——Example of transmission path effects on the spectral composition of ground motion, 1971 San Fernando earthquake. 4O NEVADA .Tnnopah Nevada/ 6 Test Si! I .Las Vegas 38 NEVADA_ UTAH Nevada Test Site 53-3" NRDS ?rea ARlZUNA O 50 KILOMETERS 116° FIGURE 36.—Nevada Test Site and vicinity. 100.0 I I l I II I I I I ll I_.,I I I I 5percent go, ..' , : __ damped .. ' . - '" $ : : mperial Valley (1940 I I'._ : orth-south componen‘ , .- Q0 -_ ,- - .' o . _ :1 .' ._ Q ... _ 10,0 Cannikin event radial component PSEUDO RELATIVE VELOCITY, lN CENTIMETERS PER SECOND PERIOD, IN SECONDS FIGURE 37.—Response spectra obtained from 1940 Imperial Valley, Ca1if., earthquake and Cannikin nuclear explosion. Recording sta- tions were located within 10 km of source. PROCEDURES FOR ESTIMATING EARTHQUAKE GROUND MOTIONS ferences in the transmission paths to two sites (fig. 38) (Weetman and others, 1970) or because of variations in the physical prop- erties of the local geology (fig. 39) within a D 100'0 l 1 111111, I 1 l llllll I I I III] S 5percent damped I U '_ _. LU '— .— w _ a: _ g — _ U) 5 “ 'l n “”"Y 9:321; E 10.0 :— cunns __ ,_ _ _ z — : w _ — Q ._ E — I t‘ — \ 5 NRDS / \ _ Lu “IJ > I w 1.0 :— , V __ 2 3 I Z I— —- ~ _ 5 — BEATTYJ _ LU — _ m —- o _ D D " _. L” E 0.1 l llllllll I llllllll l lllllll 0.1 1.0 PERIOD, IN SECONDS FIGURE 38.—Response spectra for two sites equidistant from energy source but on different travel paths. NEVADA Tonopah NevadaU Test Site\/ STATIONL ASTATIDN [31 = 366 m/s CHURCH" 183'" "MOTEL p1=1.7g/cm3 [32 = 1308 m/s pa = 2.19/cm3 4510-0: lllllllll I lllllll 15111515 ”.1 n: — RADIAL COMPONENT . M93" *1 ‘Fam‘a'd — o i — I , deVIatIon - E — I _ ..° —- ‘ ,I’ Mean _ 9 _ «‘5- '2 \/. ‘ u: — ‘i‘ ,I' Mean -1 standard— 2’ |.' deviation c: ‘v‘-'-‘ 5 1.0 _ _ Lu —- ,——~ .-‘ _ 33 " llllllll llllllll— 0.1 1.0 10.0 100.0 FREQUENCY, IN HEHTZ FIGURE 39.—Variability of ground motion recorded at two sites in Tonopah, Nev. CHARACTERISTICS OF GROUND SHAKING small geographic ara (Murphy, Lynch and O’Brien, 1971; Hays, 1972a, 1978). 4. The high degree of repeatability of site transfer functions for strain levels varying from 10'5 to 10'3 (Rogers and Hays, 1978; Hays and others, 1979). 5. The high degree of confidence with which a nu- clear explosion can be used to “calibrate” the seismic attenuation and local ground re- sponse for a region where ground-motion data are limited (Foote and others, 1970; Hays, 1975b). Newmark (1974) has reported on two little-known facts about nuclear explosion ground motions. The quantity ad-v2 (where a is peak acceleration, v is peak velocity, and d is peak displacement) has a median value of 5 to 6, and the ratio of the peak ground velocity to the peak ground acceleration is 122 cm for 1 g accel- eration (except for very close distances to a nuclear explosion). Both of these values are essentially the same as those for earthquake ground motions. INTENSITY In earthquake-resistant design, the accepted prac— tice is to express the Modified Mercalli intensity at the site in terms of peak ground acceleration in order to scale the design response spectra. This step, which must be performed with care, is needed because no pro- cedure exists for scaling spectra directly in terms of intensity. To perform the conversion, one of the empiri- cal intensity-to—acceleration relations shown in table 17 is used. Figure 40 shows two of the empirical rela- tions (Neumann, 1954; Gutenberg and Richter, 1956) 41 MODIFIED MERCALLI INTENSITY SCALE GROUND ACCELERATION,INg | ——1 —1 I III 0.005 __ 0.01 — IV 0.01 —:- _ _I v _ _ 0.05 —— VI __ _ 0.05 _ 0.1 v11 0.1 J _ vm _ — _ 0.5 -— IX 0 5 — "" I 1.0 —— _. X 10 —— Gutenberg and Richter (1956) Neumann (1954) FIGURE 40.—Intensity and acceleration relations proposed by Neumann (1954) and Gutenberg and Richter (1956). TABLE 17.—Characteristics of the data samples used in selected studies of the correlation of Modified Mercalli intensity and peak ground acceleration [Modified from O’Brien, Murphy, Lahoud (1977)] Number and location Range of location Number of Modified Mercalli Distance Acceleration Study of earthquakes recordings intensity range (km) range (cm/5’) Gutenberg and ________________________ 61, Western 167 III—VIII 3—450 1—300 Richter, 1942, United States. 1956 Neumann, 1954 ________________________ 10, do. 10 VrVIII Averages of 40—300 25 and 160 (distance dependent) Hershberger, 1956 ____________________ 60, do. 108 IleVIII ...... 1—300 Coulter, Waldron ________________________ , do. _____ IV—X Short distance 6—3000 and Devine (Not based (Dependent on 1973 entirely on Slte eology observed data) and ocal amplification) Trifunac and __________________________ 57, do. 187 IV—X 3—250 7—1150 Brady, 1975c 42 PROCEDURES FOR ESTIMATING EARTHQUAKE GROUND MOTIONS that have been used to convert intensity values into ground acceleration values. This conversion, depend— ing on the empirical relation used, can lead to a range of answers that differ by more than an order of mag- nitude and significantly affect the seismic design. The reason for the variability is that Modified Mercalli in— tensity values are a function of other variables besides peak ground acceleration. Using data from the Western United States, Trifunac and Brady (1975c) proposed empirical rela- tions between Modified Mercalli intensity [MM and: (1) peak horizontal (A h) and vertical (Av) ground accelera- tion; (2) peak horizontal (V,,) and vertical (Vu) ground velocity; and (3) peak horizontal (Dh) and vertical (D,.) ground displacement. These relations are: log A, = —0.014+0.30 [MM 10g A17 = "0.18+0.30 IMM log V,, = —0.63+0.25 IMM log V, = — 1.10+0.28 [MM 10g Dh : _O..53+019 IMM log a = - 1.13+0.24 [MM The units of acceleration, velocity, and displacement are, respectively, centimeters per second per second, centimeters per second, and centimeters. The range of [MM is from IV to X. The average trends and the standard-deviation error bars for these relations are shown in figure 41. Murphy and O’Brien (1977) derived statistical corre- lations between horizontal and vertical ground accel— eration and Modified Mercalli intensity using a worldwide data sample. The basic relations and the geometrical standard deviation (0) are: log A, = 0.241,“, + 0.26 0' = 2.19 log A, = 0281",", — 0.40 0' = 2.53 PROBABILISTIC ESTIMATES OF PEAK GROUND ACCELERATION The ground shaking hazard map of the United States (fig. 42) published in 1976 by Algermissen and Perkins provides a basis for estimating peak ground accelera- tion for a site. Unlike past maps (for example, Alger— missen, 1969) that were based on a mapping of Modified Mercalli intensity, the new map depicts the 90-percent probable peak horizontal ground accelera- tion expected at a site located on rock within a 50-year period of time. Their map was based on knowledge of: (1) the regional geology, (2) the earthquake history, and (3) the Schnabel and Seed (1973) and the “modified Schnabel and Seed” acceleration attenuation functions (see preceding section). A 90-percent probability of not being exceeded in a 50-year time interval is equivalent to a mean return period (recurrence interval) of 475 years, or an annual risk of 0.002 events per year. It should be emphasized that the earthquake causing the extreme level of ground motion may occur once or twice, or may not occur at all in 475 years; on the average, a particular level of ground motion will be exceeded once in 475 years. The Algermissen and Perkins map has only limited value for some design applications. For example, the nuclear power plant has a design requirement for peak rock accelerations with a 1,000,000 year return period. Such a map has not been constructed because of the short seismicity record and the lack of precise knowl- c: F _ I I I I I | z . o D o Vemcal component a g 0 Horizontal component U) U (I: 0: Lu n: in a: L” 3. 1000 — — n: "m ‘ f - '5 100 — - s E C E U) E c: 5 E E: . If I: ' I U T °' _ _ 2 - F 53100— pf? — E '0 .36} -. 10- Ilb ' — 0 en T o I I- r l- ‘1: u: I I z I l I 2 I1 2 W I L” _ I Lu ‘ I in. q 4 I U I 11 9 J I 5 "¢ I Z I I l E | I I u.I I '.- E i c. I I >-‘ C I I_ I : gig?“ c l < 10 _ : - I: I- I .I I ‘ d 1’ I _ E J.— ‘-’ J I '4’ ' i “ r. 3 ' D . In I a > : I < ' I I II III IV V VI VIIVIII IX X XIXII MODIFIED I II III IV V VI VII VIII IX X XI XII MERCALLI I II III IV VVI VIIVIII IX X XI XII INTENSITY FIGURE 41.—Mean values and standard-deviation error bars of peak ground acceleration, peak ground velocity, and peak ground displace- ment as a function of Modified Mercalli intensity, Western United States (modified from Trifunac and Brady, 1975c). DESIGN RESPONSE SPECTRA FOR SITE 43 125 120° 115° 110° 105° 100° 95° 90° 85° 80 75° 70 45° olgl'oya 40° 35°_ 30° EXPLANATION 4 Contour showing expected peak horzontal ground acceleration expressed as a percentage of gravity 0 Numbers within closed can tours are expected maxima. 25 ~ Maximum acceleration within the 60-percentcantour along the San Andrea: and Gar/ask faults in California is 80-09mm! of 9, using the attenuation curves of Schnabel and Seed, 1973 o 500 KILOMETERS l_|__J.___J_l__l 0 115° 11o 105° 100° I 95° 90° 85° 80° 75° FIGURE 42.—-Leve1s of peak horizontal ground acceleration expected at rock sites in the United States (from Algermissen and Perkins, 1976). The contoured acceleration values represent the 90-percent probability level; in other words, there is a 10—percent chance that these values will be exceeded within a 50-year period. edge about regional seismic attenuation in various geographic provinces of the United States. EFFECTIVE PEAK GROUND ACCELERATION An effective peak ground acceleration value (Pfossel and Slosson, 1974) is sometimes selected for earth- quake-resistant design instead of the actual value of peak acceleration. The effective peak acceleration can be thought of as the peak ground acceleration after the accelerogram has been filtered to remove the very high frequencies that have little influence on structural re- sponse. This choice is made when the peak ground ac- celeration is associated with one or more high- frequency spikes of short duration or when the total duration of the accelerogram is short. The accelero- gram produced by the 1966 Parkfield, Calif, earth- quake is one example (fig. 43) where the peak ground acceleration of about 0.5 g was caused by a single high-frequency spike. In this case, the effective peak acceleration was about 0.1 g. The accelerogram re- corded at Pacoima Dam (fig. 27) from the 1971 San Fernando earthquake has been assignedan effective peak acceleration of 0.7—0.8 g by some investigators (for example, Bolt, 1972) because the peak ground ac- celeration of 1.2 g was caused by a high frequency spike. At the present time, the use of effective peak acceleration in seismic design is controversial owing to inconsistent practice and vagueness in the definition of the characteristics of the filter used to obtain the effec- tive peak acceleration value from an accelerogram. A similar concept holds for effective peak velocity, but it is not yet well established. DEFINE DESIGN RESPONSE SPECTRA FOR SITE Once the characteristics of ground shaking at the site have been established, elastic response spectra can be defined. Response spectra are defined by using site- independent or site—dependent procedures. These pro- cedures were developed primarily for the siting of nu- clear powerplants and will be discussed in some detail below after a summary of siting procedures for nuclear powerplants. 44 PROCEDURES FOR ESTIMATING EARTHQUAKE GROUND MOTIONS 05 N. 65° E. COMPONENT ACCELERATION VELOCITY CENTIMETERS PER SECOND DISPLACEMENT 0 A M a, ~10 — — 0: Lu 5 —2o - - E -30 - 5 SECONDS _ Z I—.L_l__.L—l_l Lu 0 —40-r- -4 FIGURE 43.—Acce1eration, velocity, and displacement seismograms from the 1966 Parkfield, Calif. earthquake recorded at station Cholame-Shannon No. 2. SUMMARY OF PROCEDURES FOR SITING OF NUCLEAR POWER PLANTS The US. Atomic Energy Commission proposed seis- mological and geologic criteria in 1971 and 1973; and, under its new name, the US. Nuclear Regulatory Commission, proposed a standard review plan in 1975 for determining design earthquakes at a proposed nu- clear power plant site. These criteria significantly af- fect power plant siting and all other engineering appli- cations that require characterization of the earthquake ground motion. The procedure can be summarized as follows. The parameters of two design earthquakes, the safe shut- down earthquake and the operating basis earthquake, are estimated on the basis of integrated geologic, geophysical, and geotechnical investigations which in- clude: 1. Determination of the lithologic, stratigraphic, and structural geology of the site and sur- rounding region. 2. Identification of tectonic structures underlying the site and surrounding region. 3. Determination of physical evidence concerning behavior of the surficial unconsolidated ma- terials and the substrata during past earth- quakes. 4. Determination of the static and dynamic prop- erties of the materials underlying the site. 5. Determination of the historical seismicity. 6. Correlation of epicenters or regions of highest Modified Mercalli intensity for historically reported earthquakes with tectonic struc- tures, any part of which is located within 322 km (200 miles) of the site. Epicenters or re- gions of highest intensity that cannot be rea- sonably correlated with tectonic structures are identified with tectonic provinces. 7. Determination of the capability of each fault or part of a fault system located within 322 km of the site. 8. Estimation of the maximum magnitude earth- quake consistent with each active fault in the region. The current procedures for defining the seismic input and site response of a nuclear power plant are based primarily on empirical data. For sites in the Eastern United States, the historic earthquake that caused the largest Modified Mercalli intensity in the tectonic province containing the site is generally used to define the seismic parameters of the safe shutdown earth- quake (SSE). If this earthquake occurred in the tec- tonic province containing the site, the SSE is assumed to occur near the site and to reproduce its epicentral intensity 1,, at the site. The epicentral intensity is con- verted into a peak ground acceleration value using empirical correlations between Modified Mercalli in- tensity and peak ground acceleration. This value of peak ground acceleration (or alternately a value of ef- fective peak ground acceleration) is used to define the anchor of the high-frequency end of a smooth, broad- band response spectrum. This design spectrum is a function of damping and has a shape and amplitude level that are based on the mean-plus-one—sigma level of ground motion spectra from a number of past earth- quakes. A design time history may also be derived with the constraint that it produces a response spectrum which envelops the smooth, broad-band design re- sponse spectrum. The operating basis earthquake (OBE) is also defined and typically has seismic design values that are one-half those of the SSE. When the historic earthquake producing the largest intensity in the tectonic province lies outside the tectonic province containing the site, the SSE is assumed to occur on the province boundary at the closest point to the site. The epicentral intensity is attenuated to the site using em- pirical relations between Modified Mercalli intensity and epicentral distance that are applicable for the tec- tonic province. This value of Modified Mercalli inten- sity is then converted into peak ground acceleration and used to define the high-frequency anchor of the design response spectrum. DESIGN RESPONSE SPECTRA FOR SITE 45 For sites in the Western United States where the tectonic faults and structures are comparatively easier to identify than in the Eastern United States, the pro- cedure is similar. All the tectonic structures near the site are evaluated. The largest earthquake that has occurred anywhere on each of the nearbyfaults is de- termined and assumed to reoccur on that part of the capable fault that is closest to the site. Thus, the maximum epicentral intensity L, and the minimum epicentral distance R are defined, and epicentral inten- sity is attenuated to the site using applicable attenua- tion relations and converted into a peak ground accel- eration. Alternately, empirical relations that relate magnitude and fault rupture length can be used to define the upper-bound magnitude. This value can be converted into a value of peak ground—acceleration and attenuated to the site using peak ground-acceleration curves inferred or derived from strong ground motion data. Definition of the seismic input and site response is a controversial process. The controversy is caused in part by the debate about whether the available geologic, geophysical, seismological, and geotechnical data are adequate to specify the seismic input and site response precisely. Controversy also centers on whether the judgments about conservatism in the seismic design specifications are reasonable in view of the uncertain- ties in the data and whether a given earthquake- resistant design will provide an adequate margin of safety in a future earthquake. Perhaps the most controversial part of defining the seismic input is the question of how intense the peak ground acceleration should be, especially for the less seismic regions of the United States. The controversy is fed, at present, by the fact that: 1. peak ground acceleration is not a simple func- tion of Modified Mercalli intensity, 2. peak ground acceleration observed in an earth- quake can vary by an order of magnitude at any given distance from the source, and 3. peak ground acceleration within 10 km of the causative fault is independent of magnitude for 4.5g MS 7.1 and is a function of the dynamic stress drop and the local distribu- tion of stress. Several factors are intentionally incorporated into the design process to introduce conservatism in the seismic input for a nuclear power plant. They are: 1. selecting a low-probability, extreme event and moving it to the closest epicentral distance to the site, 2. using smooth, broad-band, mean-plus—one- sigma response spectra, independent of the epicentral distance from the site, 3. using “worst-case” seismic attenuation func- tions, 4. requiring that the design time histories pro- duce spectra that envelop the smooth, broad-band, mean-plus—one-sigma response spectra. 5. assuming that the two horizontal-component design time histories have equal values of peak ground acceleration; also, that the ver- tical component has peak values that are 2/3 or more of the peak values of the horizontal components, and 6. modifying the smooth, broad-band, mean-plus- one-sigma response spectra to account for specific local ground response characteristics. The safe shutdown earthquake is assumed to repre- sent the maximum possible level of earthquake ground shaking at the site. The SSE may or may not have occurred at the site during historic times, but it is de- fined on the basis of a detailed investigation of the regional and local geology, the regional seismicity, and the characteristics of the underlying soil materials. For the SSE, the facility should be designed so that all systems necessary to protect the health and safety of the public will remain functional both during and after the earthquake. Although structures and their inter- nal components may suffer severe damage from the SSE, the design must allow for a safe and orderly shut- down after an earthquake. The lower bound for the peak horizontal ground acceleration induced at the site by the SSE is 0.1 g. The second level, the operating basis earthquake is assumed to represent the maximum level of ground shaking that can be expected to occur at the site during the 40-year operating life of the nuclear power plant. This earthquake probably has occurred in the vicinity of the site during historic times. It is also based on a detailed investigation of the regional and local geology, the regional seismicity, and the characteristics of the underlying soil materials. For the OBE, the facility should be designed so that those features of the plant necessary for continued operation without undue risk to the public will remain funtional. The maximum horizontal ground acceleration of the OBE must be at least one-half of the SSE, with a lower-bound peak acceleration level of 0.05 g. Table 18 lists the horizontal ground accelerations of the OBE and SSE for a number of nuclear power plant sites. To obtain information for evaluating the ground re- sponse in the site vicinity, a combination of borings, trenches, laboratory measurements, and geophysical methods are used (Shannon and Wilson and Agbabian Associates, 1972, 1975, 1976). The purpose is: (1) to determine the classification, lateral distribution, stratification, geologic structure, and physical prop- 46 PROCEDURES FOR ESTIMATING EARTHQUAKE GROUND MOTIONS TABLE 18—Horizontal ground accelerations for the operating basis earthquake and safe shutdown earthquake for nuclear power plant Sites in the United States [From Johnson and Kennedy (1977)] Operating Safe- basis shut down earthquake earthquake Site State g leve g level Browns Ferry ____________ Alabama 0.10 0.20 Brunswick ________________ North Carolina .08 .16 Calvert Cliffs ,,,,,,,,,,,,,, Maryland .08 .15 Connecticut Yankee ________ Connecticut __ r_ .17 Davis- Besse ________________ Ohio .08 .15 Dresden ,,,,,,,,,,,,,,,,,, Illinois . 10 .20 Fermi ____________________ Michigan .08 .15 Fort St. Vrain ______________ Colorado .05 .10 Indian Point ,,,,,,,,,,,,,, New York .10 .15 Kewaunee ................ Wisconsin .06 .12 Nine Mile Point ____________ New York .07 .11 Oyster Creek ______________ New Jersey .11 .22 Peach Bottom 11111111111111 Pennsylvania .05 .12 Quad Cities ________________ Illinois .12 .24 Salem ,,,,,,,,,,,,,,,,,,,, New Jersey .10 .20 Susquehanna ______________ Pennsylvania .08 . 15 Three Mile Island d . .06 12 Turkey Point _ 1-- .03—.05 15 Vermont Yankee .07 .14 Zion ______________________ Illinois .08 .17 Diablo Canyon ,,,,,,,,,,,, California .33 .66 Humboldt Bay ,,,,,,,,,,,, do. .25 40 Rancho Seco ______________ do .33 66 San Onofre ,,,,,,,,,,,,,,,, do. .66 .75 Trojan ____________________ Oregon .15 .25 WPJPSS (Hanford) __________ Washington .13 .25 erties of the unconsolidated materials (soil) and rock underlying the site; (2) to obtain samples and cores for laboratory testing; and (3) to establish the elevation and variation of the ground-water table for foundation studies. The response of geologic materials to ground shaking is governed primarily by: (1) shear wave velocity, (2) density, (3) shear modulus, (4) material damping prop- erties, (5) Poisson’s ratio, (6) bulk modulus, (7) static shear strength, and (8) dynamic shear strength. These physical properties are determined from laboratory and field tests and are used in conjunction with finite- element and other computational models to calculate the ground response for the proposed site (Seed and Idriss, 1969; Joyner and Chen, 1975; Joyner, 1975). The current procedures for specifiying the earth- quake ground motion at a nuclear power plant site are based primarily on empirical models. Each empirical model is based on the collection and use of catalogs of seismograms, response spectra, and observational data (for example, intensity data) from past earthquakes. These data are used to derive earthquake recurrence relations, scaling laws, attenuation functions, and to confirm the adequacy of data analysis procedures. The deterministic method uses analytical models to simu- late earthquake source mechanics, transmission path effects, and local ground response. In principle, analyt- ical models can be applied any place in the world; the only requirements are that the proper earthquake source, transmission path, and ground response models be selected. The analytical approach has not been ac- ceptable in earthquake-resistant design in the past be- cause the physical processes that occur in the source, path, and site are not well understood. At the present time only local soil conditions are modeled mathemat- ically in earthquake-resistant design. The data sample is particularly limited for siting of nuclear power plants in the Eastern and Central United States. In these regions, Modified Mercalli in- tensity data extracted from the historic earthquake record may be the only data available to define the design ground motion. In these cases, an attempt is made to select conservative values. Conservative val- ues are introduced as follows: 1. The location and magnitude of likely earth- quakes are uncertain because of the short historical seismicity record; therefore, a rare large-magnitude event located close to the site is selected. 2. The site intensity (as measured by peak ground acceleration or Modified Mercalli intensity) is uncertain due to lack of knowledge about re- gional seismic wave attenuation and the local ground response; therefore, an upper-bound value is selected. 3. The response spectrum that corresponds to a given site intensity exhibits scatter about a mean value and is uncertain because the de- tails of earthquake ground can vary widely for a given measure of ground motion (fig. 44); therefore, smooth, upper-bound spectral values are used. SITE-INDEPENDENT RESPONSE SPECTRA The site-independent procedure, one of two basic methods for specifying design response spectra for nu- clear power plant sites, is based on the use of standard spectrum shapes. The standard spectrum shapes are considered to be independent of the characteristics of the site because the ensemble of seismograms from which the spectra were derived depict ground motions for a wide range of geologic and seismological condi- tions. The site-independent method was first introduced by G. W. Housner in 1959. He derived smooth normalized acceleration and velocity response spectra (fig. 45) from the two horizontal components of ground acceleration recorded at four sites from four large earthquakes in the Western United States (table 19). The earthquakes were: (1) 1934 Imperial Valley, Calif. (M =6.5); (2) 1940 Imperial Valley, Calif. (M =7.0); (3) 1952 Kern DESIGN RESPONSE SPECTRA FOR SITE 47 :I 45.7 5200 0 ' I I ' I ' I ' I f I ' I S - I I l I I I I I I I I I I | l | I I I I ' l I I I I D . a”; MODIFIED MERCALLI INTENSITY g ‘ ”“TP'WW” ' 3 100.0 _— Is VII AT EACH SITE 1 t 3,4 ”mm, m _ — 5 I 30.5 — a: — “ 0 Lu LIJ - — _I Q. 5 ' - E a E ‘ ‘ A '-,'_-‘ — E _ — é u.I Lu '- E 15.2 — — U _ _ a I- E a. 5 ~ 03 40 I: o 2 I— II] — a 10.0 _— —-I _ ll/fl’ S : : 0 [I I I I l I I I I I I I I I LU — — i — .... .. GrIffIth ParkIrock), peak —I I I I I I I I I I I I I I 2 ‘ acceleration 0.18y “ I] 62 / E ‘ __ Glendale Municipal Building ‘ ' I, Earthquake Scaling factor E _ (alluviumlupeak acceleration _ _ I Imperial Valley, Calif.. 1940 2.7 _ o ."I 0.27 y I Imperial Valley, Calif., 1934 1.9 g 1.0 I I I I I I I II I I I I I I I II I I I I I I II _I PugetSound,Wash., 1949 1'9 ' I024 0.01 0.1 1.0 10.0 _I Kern County, Calif., 1952 1.6 I _ PERIOD,IN SECONDS ‘I ‘ . _ . . I Be ' f t FIGURE 44,—Var1ation 1n ground-motlon response spectra and peak 0.31 —+ 71:23.12; or _ ground acceleration values for the same value of Modified Mercalli intensity. Spectra were derived from accelerograms of the San Fernando, Calif. earthquake (modified from Murphy and O’Brien, 1977). County, Calif. (M =7.7); and (4) 1949 Puget Sound, Wash. (M =7.1). The epicentral distances ranged from 8 to 56 km. The recording sites were underlain by rock, stiff soil, and deep cohesionless soil. These spectra are scaled by a factor based on the spectrum intensity rather than the peak ground-acceleration. In 1969, Newmark and Hall proposed a new tech- nique for estimating site-independent spectra. Their technique as based on the fact that the response spectrum over certain frequency ranges is related by an amplification factor (fig. 46) to the peak values of ground acceleration, velocity, and displacement. The amplification factors were statistically determined from response spectra derived from the accelerogram recorded at El Centro from the 1940 Imperial Valley, California earthquake (table 19) and are a function of the damping of the spectra. The factors correspond to about the 84th percentile of a log-normal distribution. In this method, the estimated values of peak ground acceleration, velocity, and displacement for the site are plotted on tripartite logarithmic paper. Using the amplification factors that correspond to the desired percentage of critical damping, the peak ground mo- tion values are amplified to give a smooth design re- sponse spectrum for the site. Newmark and Hall (1969) also proposed a “stan- dard” earthquake response spectrum for use whenever adequate detail about the site ground-motion parame- ters was unavailable. These spectra (fig. 47) are based on peak ground motion values observed at El Centro from the 1940 Imperial Valley, California earthquake, SPECTRAL ACCELERATION, IN y PERIOD , IN SECONDS FIGURE 45.—Site-independent velocity and acceleration response spectra (modified from Housner, 1959). but are about 50 percent more conservative that the El Centro values. The Imperial Valley accelerogram was recorded within 10 km of the fault, the bracketed (5 percent) duration of strong motion was nearly 20 sec- onds, and the levels of peak horizontal ground acceler- ation, velocity, and displacement were, respectively, 0.33 g, 34.8 cm/s, and 21.1 cm. The soil column at the El Centro site contained a 30-m-thick layer of stiff clay that amplified the input rock accelerations (Schnabel, Seed, and Lysmer, 1972). The proposed standard earthquake had peak ground motion values of 0.5 g, 60.9 cm/s, and 45.7 cm, but it could be scaled to any peak ground acceleration level on the assumption that the three ground-motion parameters are proportional to each other regardless of the level of ground shaking. Thus, one would use peak ground velocity and dis- placement values of 121.8 cm/s and 91.4 cm for an earthquake having a peak ground acceleration of 1.0 g. The studies by Newmark and Hall (1969) also pro- vided an empirical basis for defining vertical ground- motion spectra. They observed that the peak vertical ground-accelerations are about two-thirds of the peak PROCEDURES FOR ESTIMATING EARTHQUAKE GROUND MOTIONS 353:8 N N mm 8 mm mo xcmm Hmm. N N «H fism aH 3330 3%. mme N N wmm InonoHHoO moan mm .820 mm. N I I xoom mlm EmQ «860mm vNH 99 HhmH I I I I Emo .owcmgwm :mm mmmH N wudmm .conmHHoo waQ ooH wHHoEHHomHIH mm. ms wme IIIIIIIIIIIIIIII SEE“ .330 N mm 3. on 955.0 Hm mH. mé wme I I I I .5358: omwtom N Sam gum owH «55 NV. ms wmmH IIIIIIIIIIIIIIIIII 5.8m 9* :ouEEm N mm ow m .2529an vmm. 3x ~898an N wv aim H d .wEmHoHHO mwv. . N II Moom b 33809 Sum. Qm wme IIIIIIII szo Emwimm x 12 .3. mm «3520 mad 3 $2 I :33 Esom awmsm . www— owH .conoLoo ame CV HBEHHOHIH wH. mm. meH IIIIIIII EEO .HSmHHHoHH {an I £30 N N II xuom HH 030 H5390 moH. mad bmmH IIIIIIII oomHocgnH :wm N mm him owH 38.50 a mo. wd wmmH IIIIIIIIIIII .Emo 6.8m N owH 0H. om mHmqunH How. mmoH N N owH .nononou mama mm ~28de 3N. 9w vmmH IIIIIIIIII .Emo .mxmgdm N vw OH. mHH 8H .mH.nH .2 we. N vw bfim mH H unwfiwmmmd mo. @92on Eoizom I E8 N N II xoom om de EH4 >6 NmmH IIIIIIIIII 3560 PEN N 8H OH. mm mewEwnH H H. w.m HmmH I I «Exemzmo HmwksfioZ mme N N NmH IconcHHoo mama om mEEbO mm. HS mva IIII smug 655m awash a? N N N N mm aim w 95sz Hm mm. H: 03H IIIIII “33> Hwto EH N II moom w manoHIH 9H. 06 mmmH IIIIIIIIII E02 953: N N mm .msm mm 0.55.0 E Nde m6 vmmH IIIIII «HEofiHmO $264 85: 83: 88: 8mm: :5 :oaaofimmso :5: 8; a: magnum: sun oxmsgtam 9:35 “HI—«5302 HES HEN $5“ch x9983 :om 8:531 mnHEOuQm 5.5333on #35ka 8 5me Habamoam xwwm wHSmMSmgnH 33253535 36 HEM wxnsozfimm 48 53QO EwfifiwflwESIwfim ~5me 2 fimm: wESmemHmooa wfiszvfitamldH mam—<9 DESIGN RESPONSE SPECTRA FOR SITE VELOCITY AMPLIFICATION FACTOR SPECTRUM BOUNDS PSEUDO RELATIVE VELOCITY (LOG SCALE) a Q / $§Q~ $39qu PEAK HORIZONTAL “$3 6” GROUND MOTION 2" :3 D. PERIOD (LOG SCALE) AMPLIFICATION FACTOR 35‘3ng ACCELERATION VELOCITY DISPLACEMENT 0 6.4 4.0 2.5 0.5 5.8 3.6 2.2 l 5.2 3.2 2.0 2 4.3 2.8 2.0 5 2.6 1.9 1.8 7 1.9 1.5 1.4 10 1.5 1.3 1.2 20 1.2 1.1 1.0 FIGURE 46,—Schematic illustration of technique for developing site- independent response spectra (modified from Newmark and Hall, 1969). The quantities a, v, and d refer to the peak ground accelera- tion, velocity, and displacement; PSAA, PSRV, and RD refer to the spectral acceleration, velocity, and displacement. horizontal ground-accelerations when the fault movements are primarily horizontal, and that the ver- tical motions are approximately equivalent to the hori- zontal motions when the fault movements involve a large vertical component. Amplification factors are also applied to the estimated peak vertical ground- motion to obtain the design response spectra. Values of maximum ground velocity and displace- ment are typically based on those of the standard earthquake when specific information about these pa— rameters is unavailable. The standard earthquake has peak velocity and displacement values that are pro- portional to those obtained from the 1940 Imperial Val- ley, Calif, accelerogram (table 20) and are scaled pro- portionately up or down to correlate with the value of ground acceleration that specifies the SSE. The va- lidity of this scaling procedure has been questioned, but no alternate techniques have been adopted. The United States Atomic Energy Commission pro- posed guidelines in 1973 for developing improved site- independent earthquake response spectra. These 49 guidelines, contained in AEC Regulatory Guide 1.60 (1973b), were based on two independent studies of the statistical properties of response spectra (see table 19) of earthquake ground motions by N. M. Newmark Con- sulting Engineering Services (1973) and J. A. Blume and Associates, Engineers (1973). From the results of these two studies, a unified procedure (Newmark and others, 1973) was developed for defining site- independent earthquake—response spectra that are ap- plicable to most sites. The only exceptions are sites which are relatively close to the epicenter of a postu- lated earthquake or sites which have physical charac— teristics (for example, foundation deposits with well- defined frequency-filtering characteristics) that could significantly enhance the spectral characteristics of ground motion in a portion of the spectral band of interest. The procedure proposed in Regulatory Guide 1.60 is very similar to the one proposed by Newmark and Hall (1969). Spectrum amplification factors (table 21) are based on a much larger number of earthquake ground-motion records than used by Newmark and Hall and represent the mean-plus-one-standard— deviation statistical level (84th percentile of a log- normal distribution). To define horizontal response spectra, the peak horizontal ground acceleration and displacement levels are established. The peak horizon- tal ground displacement is considered to be pro- portional to the peak horizontal ground acceleration and is fixed at 91.4 cm for an acceleration of 1.0 g. These two values are proportional to the values of 45.7 cm and 0.5 g established for the “standard” earth- quake. The bounds of each spectrum established by five line segments, similar to the Newmark and Hall pro- cedure. The control points are designated by letters A, B, C, and D and have specified frequency ranges for all horizontal component spectra (fig. 48). The amplifica- tion factors (table 26) are a function of the percent of critical damping and are specified for each of the con- trol points. Vertical response spectra are constructed in a simi- lar manner to horizontal response spectra (fig. 49), but three differences are incorporated into the procedure: The frequency for control point C is located at 3.5 Hz rather than at 2.5 Hz, (2) The amplification factors (ta- ble 22) are different, and (3) The value of peak horizon- tal acceleration is used as the initial reference value. In the frequency range 025—35 Hz, the ratio of vertical to horizontal spectral amplitudes varies between two- thirds and one. The horizontal and vertical response spectra are identical in the frequency range 3.5—33 Hz. Beyond 33 Hz, the vertical spectral accelerations de- crease from a value equal to the peak horizontal ground acceleration at 33 Hz to two-thirds of the hori- zontal ground acceleration at 50 Hz (control point, A’). 50 PROCEDURES FOR ESTIMATING EARTHQUAKE GROUND MOTIONS PERIOD, IN SECONDS 10 5 2 I 0.2 0.1 0.05 0.02 0.01 "’0" I I | I | I I o"& \@ Damping factor (percent) ,0 0 0 0.5 ‘9 1 $0 ‘ l' 2 \Q A5 l 10 0,, _ 9e ‘6 \§ round-motion maxima 100 7 6‘ o“ .3 SPECTRAL VELOCITY, IN CENTIMETERS PER SECOND 100 FREQUENCY, IN HERTZ FIGURE 47.—“Standard” site-independent horizontal response spectra (modified from Newmark and Hall, 1969). TABLE 20.—Relatiue values of maximum ground-acceleration, veloc- ity, and displacement; "standard” earthquake [From Newmark and Hall (1969)] Maximum values of ground motion Acceleration Velocity Displacement Condition ) (cm/s) (cm) "Standard” relative values __________________ 0.5 60.96 45.72 Typical maxima Imperial Valley (1940) Horizontal ____________ .33 40.64 30.48 Vertical ______________ .22 27.94 20.32 Recommended minimum for any region: Horizontal ____________ .10 12.70 10.16 Vertical ______________ .07 7.62 7.62 TABLE 21.—Horizontal design response spectra and relative values of spectrum amplification factors for control points [From US. Atomic Energy Commission Regulatory Guide 1.60 (1973)] Percent of critical Acceleration Displacement damping A(33 Hz) B(9 Hz) C(2.5 Hz) D(0.25 Hz) 0.5 1.0 4.96 5.95 3.20 2.0 1.0 3.54 4.25 2.50 5.0 1.0 2.61 3.13 2.05 7.0 1.0 2.27 2.72 1.88 10.0 1.0 1.90 2.28 1.70 The horizontal site-independent response spectra (scaled to 0.1 g) proposed by Housner (1959), Newmark and Hall (1969), and US. Atomic Energy Comm. (1973b) are compared in figure 50. The differences be- DESIGN RESPONSE SPECTRA FOR SITE 51 PERIOD, IN SECONDS 5.0 2.0 1.0 0.5 0.2 0.1 0.05 0.02 0.01 1000‘ - factor (perc' 100 SPECTRAL VELOCITY, IN CENTIMETERS PER SECOND 0.3 0.1 100 FREQUENCY, IN HERTZ FIGURE 48.—Site-independent horizontal response spectra scaled to 1.0 g. See table 21 for amplification factors at control points A—D. Modified from US. Atomic Energy Comm. Regulatory Guide 1.60 (1973b). tween the Newmark and Hall and the AEC Regulatory Guide 1.60 spectra are very small. The Housner spectrum, however, differs significantly from the other two design spectra. This difference is due to the pro- bability level associated with each of the three ap- proaches and the differences in the data samples. The Housner spectrum is an average derived from eight horizontal accelerograms; therefore, it would normally be exceeded 50 percent of the time. The Newmark and Hall and Regulatory Guide 1.60 spectra were derived from a much larger data sample and represent the mean-plus-one-standard-deviation probability level, or the 84th percentile; therefore, it should beexceeded about 16 percent of the time. In addition, the Newmark-Hall and Regulatory Guide 1.60 spectra were based on ultimate strength design concepts in- stead of the design practice in effect during the time the Housner-type spectra were developed which utilized working stress concepts with allowable stress- es being one-third less yield. A more accurate compari- son is obtained when the Housner spectrum is in- creased by 50 percent. In this case, the agreement of all three spectra is good, especially in the 2—10 Hz range that is important in nuclear power plant design. SITE-DEPENDENT RESPONSE SPECTRA The current library of strong-motion accelerograms, although limited, contains representative samples of a wide variety of local site conditions (see tables 15, 16). 52 PROCEDURES FOR ESTIMATING EARTHQUAKE GROUND MOTIONS PERIOD, IN SECONDS 5.0 2.0 1.0 0.5 0.2 0.1 0.05 0.02 6‘ “Q9 .3 <\\ S ®% 1000 e '8‘, D E Damping factor (percent) a 0.5-\ | 5 l B l \ U) S ‘ | I E | (.9 Q '— 100 2 <25 \ | 3 6° I E «S I | f." _ l s a y a w > ._J < | n: '— | U E 9’ 4(4 1 0 . 0Q 0o ‘9 C ) ‘7» 10 /0 i v ¢ 9/ I . 9 ”ix I ED %‘I’¢>\ | 0"? 0909g®;\ i ’5’; ’00 \i ’o ”0' §~ 3 0.1 0.2 0.5 1 Z 5 10 20 50 100 FREQUENCY, lN HERTZ FIGURE 49.—Site-independent vertical response spectra. See table 22 for amplification factors at control points A’—D. Modified from US. Atomic Energy Comm. Regulatory Guide 1.60 (1973). TABLE 22,—Vertical design response spectra and relative values of spectrum amplzfication factors for control points [From US. Atomic Energy Commission Regulatory Guide 1.60 (1973b)] Percent of critical damping Acceleration Displacement A'(50 Hz) A(33 H2) B(9 Hz) C(3.5 Hz) D(0.25 Hz) 0.5 0.67 1.0 4.96 5.67 2.13 2.0 .67 1.0 3.54 4.05 1.67 5.0 .67 1.0 2.61 2.98 1.37 7.0 .67 1.0 2.27 2.59 1.25 10.0 .67 1.0 1.90 2.17 1.13 In some cases, it is possible to select an ensemble of response spectra that were derived from seismograms whose local site conditions (Duke and Leeds, 1962; Matthiesen and others, 1964; Woodward-Lundgren and Associates, 1973; Shannon & Wilson and Agba- bian Associates, 1976; Gibbs and others, 1976) either match or are very similar to those of the proposed con- struction site. Spectra meeting this condition satisfy the seismic and geologi . criteria proposed in 1971 by AEC for nuclear powerplant sites more easily. To date, site-dependent response spectra have not been used ex- tensively in power plant siting because of the limi- tations of the ground-motion data sample. The site-matched seismograms and response spectra should also match the source-mechanisms and the transmission-path characteristics of the designated de- DESIGN RESPONSE SPECTRA FOR SITE 53 PERIOD, lN SECONDS 100 10 1 0.1 0.01 100 10 —é¢ SPECTRAL VELOCITY, IN CENTIMETERS PER SECOND Note: Damping factor 5 percent; spectra normalized to zero- period acceleration of 0.1 g 0,3 1 Illllll 0.01 0.1 FREQUENCY, IN HERTZ FIGURE 50,—Comparison of site-independent horizontal response spectra produced by three different procedures. Sign earthquake if possible. The ideal case is to have an ensemble of seismograms and response spectra that closely match the source-path-and—site parameters for the proposed construction site. The limits of the data sample usually prevent matching any of the parame- ters except those associated with the site. Seed, Ugas, and Lysmer (1976) studied a selected set of 104 accelerograms (see table 6—9) to establish site- dependent response spectra for four site classifications. Their classifications were: (1) rock (28 records), (2) stiff soil (31 records), (3) deep cohesionless soil (30 records), and (4) soft to medium soil (15 records). In their study, ensemble average and mean-plus-one-standard— deviation response spectra were derived for each site classification. These spectra, normalized to 0.1 g at zero-period, are shown in figure 51. Site-dependent spectra exhibit the following frequency-dependent effects. The spectra from sites underlain by soft to medium soil and deep cohesionless soil have larger amplitudes than the spectra from sites underlain by rock and stiff soil for low frequencies (less than 1—3 Hz). For higher frequency responses (above 6 Hz), the spectra from rock and stiff soil sites exhibit larger amplitudes than the spectra for deep and soft site conditions. The mean-plus-one-standard-deviation site- dependent response spectra are compared in figure 52 with the spectrum developed on the basis of ABC Regu- latory Guide 1.60. This comparison shows that differ- ences are most pronounced for frequencies below 2—3 Hz. For sites underlain by stiff soil or deep cohesionless soil, higher frequency components of the site- dependent spectra differ from Regulatory Guide 1.60 spectra by about 25 percent. The use of site-matched seismograms to develop site-dependent design response spectra is superior to 54 PROCEDURES FOR ESTIMATING EARTHQUAKE GROUND MOTIONS 4 I I I I I 5 percent dampEd DEEP COHESIONLESS SOIL Mean +1 standard deviation | I | | I 5 percent damped SOFT T0 MEDIUM STIFF CLAY Mean +1 standard deviation SPECTRAL ACCELERATION MAXIMUM GROUND ACCELERATION 5 percent damped 5 percent damped ROCK STIFF CLAY Mean +1 standard deviation 3.0 0 0.5 3.0 PERIOD, IN SECONDS FIGURE 51.—Site-dependent mean and mean-plus-one-standard-deviation response spectra for four site classifications (modified from Seed, Ugas, and Lysmer, 1976). Spectra are normalized by dividing by peak acceleration. site-independent procedures. Unlike the site- independent procedures, site-dependent procedures will generally produce ground-motion parameters that correspond closely with those expected on the basis of the seismological and geologic conditions at the site. The limiting factor, however, is that the source mecha- nisms, transmission-path characteristics, and local ground response may not be fully represented by the available data. Also, the ensemble of relevant seismo— grams may be inadequate to permit meaningful statis- tical evaluations of the variance in the data. For these reasons, care must be exercised to ensure that the most reasonable procedure for defining the design response spectrum is used. DESIGN TIME HISTORIES Time histories of particle acceleration, velocity, and displacement are the most accurate representations of the seismic input. These representations are not needed for all earthquake-resistant design applica- tions, but they are frequently used when analyzing the response of important structures such as nuclear power plants. At the present time, no model is completely adequate for deterministically computing the acceleration time history of ground motion for arbitrary source-site configurations. Current design practice, therefore, requires design time histories to be developed in con- junction with the design response spectrum. The time histories are constrained to be compatible with the de- sign spectrum for the site and to account for the peak ground shaking parameters, spectral intensity, and duration of shaking. Compatibility is defined to mean that the envelope of all the response spectra derived from the time histories lies above the smooth design response spectrum throughout the frequency range of interest. DESIGN RESPONSE SPECTRA FOR SITE PERIOD, IN SECONDS 100 10 1 0.1 0.01 0 10 «90 6‘0) ’2? $6“ Damping factor 5 percent; 0 spectra normalized to Q/ 06‘ zero-period accelera- $3,“ (6;, " tion of 0.1 g Q, 4&0 4: ’4’ 10 EXPLANATION Ensemble average: .. .. . .. Soft-medium soils _. _ Deep cohesionless soils ____ Stiff soil deposits _ _ _ Rock 0.5 100 Damping factor 5 percent; . spectra normalized to 1. zero-period accelera- -. tion of 0.1 g SPECTRAL VELOCITY, IN CENTIMETERS PER SECOND EXPLANATION Ensemble mean plus one standard deviation: .. .... Soft-medium soils __._ Deep cohesionless soils \\ _-_ Stiff soil deposits __.... Rock «Q‘Q '. Regulatory Guide 1.60 . 1 "\ 6‘ . 6x“ §. 0 0.5 0.01 0.1 1 10 100 FREQUENCY, IN HEHTZ FIGURE 52.—Comparison of site-dependent mean and mean-plus-one-standard-deviation response spectra with AEC Regulatory Guide 1.60 spectrum. 56 PROCEDURES FOR ESTIMATING EARTHQUAKE GROUND MOTIONS Several techniques have been proposed for generat- ing time histories (for example, Bycroft, 1960; Housner and Jennings, 1969; Rascon and Cornell, 1969; Lynch and others, 1970; Seed and Lysmer, 1972). These tech- niques can be categorized as follows: 1. Use of empirical data. Strong-motion accelero- grams of past earthquakes, recorded either in the vicinity of the site or at other locations that have similar source-path-site charac- teristics, are used with appropriate modifica— tion to develop a design time history for the site of interest. 2. Use of calculational models. Because earth- quake accelerograms recorded at moderate distances have the appearance of random time functions, calculational models have been developed that use white noise, filtered white noise, and stationary and nonstatio- nary filtered white noise to generate design time histories. The present lack of an adequate ensemble of strong- motion accelerograms is the major limitation in devel- oping design time histories. DETERMINE LOCAL GROUND RESPONSE It has been recognized and widely documented since the early 1900’s that the physical properties of the geologic materials underlying the recording site can significantly modify the amplitude level and spectral composition of the ground motion recorded there. Structures founded on unconsolidated materials are frequently damaged (Page, Blume, and Joyner, 1975; Steinbrugge and others, 1975) by ground shaking, and the damage distribution on many occasions has been recognized to be related to the site. Buildings of a cer- tain class or type (that is, having a certain natural period) are often damaged from ground shaking when located on geologic materials having a similar charac- teristic site period, whereas buildings with a different natural period located on the same foundation mate- rials are not damaged. The distribution of damage is explained by the fact that the local geologic materials amplify the ground motion input in a period range that coincides with the natural period of vibration for the damaged structure. When the unconsolidated material underlying a re- cording site modifies the seismic input, the amplitude of the surface ground motion increases in a narrow range of frequencies and decreases for other frequen- cies. The amplitude of the amplified ground motion is a function of the shear wave velocity, the density and material damping, the thickness, water content, and where the surface or bedrock topography is irregular, the geometry of the unconsolidated deposits and under- lying rock. The frequency range that is affected is a function of the thickness and physical properties of the unconsolidated materials (Seed and Idriss, 1969; Mur- phy, Weaver, and Davis, 1971; Joyner and Chen, 1975; Borcherdt and Gibbs, 1976; Hays, 1977a, 1978). The amplification effect for surface waves is similar to that of body waves except that the thickness of the unconsolidated materials also affects the magnitude of amplification (Murphy and Davis, 1969; Drake and Mal, 1972; Hanks, 1976). Local ground response is complicated by the fact that unconsolidated materials behave nonlinearly. Soils have a shear modulus and damping characteristics that are strain dependent (fig. 53). Soil nonlinearities and inelasticity may attenuate, rather than amplify, the surface ground motions relative to the rock ground motions under certain conditions. The topography of the site has also been demon- strated to have an important frequency-dependent ef- fect on ground motion (Bouchon, .1973; Trifunac, 1973a; Rogers and others, 1973; Boore, 1973; Wong and others, 1977). Figure 54 shows the variation in terms of smooth response spectra that would be expected if the same earthquake was recorded on stiff, medium, and soft soil columns (Smoots and others, 1969). These spectra show that the high-frequency components are usually damped out fairly rapidly by thick soft soils, reducing the level of peak ground acceleration. 30 l 20 fl / WWW 10-2 w" 100 DAMPING RATIO, IN PERCENT EARTH OUAKES AND NUCLEAR EXPLOSIONS FAILURES surface rupture) Surface vibrator LABORA column 1044 PERCENT |—Torsiunal EQUIVALENT MDDULUS AT SHEAR STRAIN 7 EQUIVALENT MODULUS FOR7 Cyclic simple shear 10-4 10-3 10‘2 10'1 10° SHEAR STRAIN, IN PERCENT FIGURE 53.—-Effect of strain level y on shear modulus and damping of soils (from Seed and Idriss, 1969). DETERMINE LOCAL GROUND RESPONSE 57 PERIOD, IN SECONDS 10 SPECTRAL VELOCITY, IN CENTIMETERS PER SECOND I IIIIII 0.1 1.0 FREOUENCY, IN HERTZ 10 100 FIGURE 54.——Comparison of smooth response spectra for three soil columns (modified from Smoots and others, 1969). From the present United States earthquake data sample, 104 horizontal-component accelerograms (see table 15) can be grouped in four broad site class- ifications: (1) rock or rocklike material having a shear wave velocity at low (0.0001 percent) strains of at least 760 m/s, (2) stiff soil (firm soils less than about 45 m thick), (3) cohesionless soil (sandy soils exceeding 80 m in thickness), and (4) soft to medium-stiff clay. Seed, Ugas, and Lysmer 1976) analyzed the horizontal accel- eration spectra representing these four site classifications and determined normalized average spectral response for each classification (fig. 55). Significant amplification relative to rock is indicated for sites underlain by soft-to-medium clay and sand- sites for periods greater than 0.5 second. Empirical data showing the effect of site geology on body waves have been obtained from the nuclear explo- sion safety program at the Nevada Test Site (Murphy, Weaver, and Davis, 1971; Hays, 1972a, b; Hays, 1978) and earthquake aftershocks (Murphy, Lynch, and O’B- rien, 1971; Hays, 1977a). A classic example of body- wave amplification is illustrated by ground-motion data recorded at Tonopah, Nev. (fig. 39). A pair of seis- mograph stations located 183 m apart and underlain 4 I I | I I Total number of records analyzed: 104 Spectra for 5 percent damping Soft to medium clay and sand; 15 records Deep cohesionless soil (>80 m); 30 records Stiff soil (<45 m); 31 records Rock; 28 records SPECTRAL ACCELERATION MAXIMUM GROUND ACCELERATION N | PERIOD, IN SECONDS FIGURE 55.—Average acceleration response spectra for four site classifications (from Seed, U gas, and Lysmer, 1976). 58 PROCEDURES FOR ESTIMATING EARTHQUAKE GROUND MOTIONS respectively by dacite, an igneous rock, and 13.1 m of mine tailings (fill) have recorded input ground motions with a dynamic range of 10—5—10‘3 g from more than 20 nuclear explosions located about 100 km away. The low acoustic-impedance contrast of the fill relative to rock causes the ground-motion spectral components at around 7 Hz to be amplified by a factor of six. The mean site transfer function is repeatable from event to event with a standard deviation (0') of 1.30. The level of peak ground acceleration is also larger by a factor of about two at the station underlain by fill. The strain levels induced in the fill were low (10’5 to 104) because of the large source-to-recording site distances; therefore, the fill responds almost elastically to the input ground motions. A second example of body wave amplification was observed in the Glendale, Calif. area. Mea- surements of ground motion from the aftershocks of the 1971 San Fernando earthquake were made at a number of locations in San Fernando Valley. Two sta- tions in Glendale (fig. 56) exhibit significant relative ground response. The mean site transfer function is repeatable from event to‘event with a standard devia- tion (0') of 1.50. Las Vegas Valley in southern Nevada is underlain by Cenozoic alluvium of widely varying thickness (Di- ment and others, 1961; Longwell and others, 1965). It has been recognized since 1970 (Davis and Lynch, |———— 3.2km ——| Station 38 Station 41 ”Fl ’32 = 1900 m/s p2 =2.0 g/cm3 8.0 _ I I I I I l I I I I _ — TRANSVERSE COMPDNE T — g g 51, _ standard _ z z _ iation _ o o I: I: — /\ — < 1 s) and essentially uniform response at short periods (0.1-0.5 s), suggests that the principal effects are caused primarily by long-period surface waves propagating unevenly in the variable surface alluvium. The ground response derived from the radial component of spectral velocity for the long-period band 3.33—4.50 s correlates fairly well with the estimated thickness of the al- luvium in Las Vegas Valley (fig. 58). How ground motion varies with depth is not well known. One of the fairly recent studies (Murphy and West, 1974) used identical seismograph systems emplaced in tuff at the bottom of a 41-m drill hole at Beatty, Nevada and at the surface on alluvium. A large number of nuclear explosions were recorded on this array and established the repeatability of the site transfer function (fig. 59) for low-strain levels. The amplitudes of the spectral components, especially those in the 0.05—0.5 second range, decrease with depth. The most significant decrease occurs for periods in the vicinity of 0.3 second, the characteristic site period (that is, the period that corresponds to four times the thickness of the alluvium divided by its shear wave velocity). This experiment also demonstrated that the input ground motion at the base of the alluvium is essentially identical (except for the free-surface effect) to the ground motion observed at a surface site located on rock. The limited data available at present are inadequate to prove the common assumption of a broad-band spectrum at depth. The present lack of data to show how ground-motion time histories and their response spectra change with depth is a major source of uncertainty in some earth- quake-resistant design engineering applications such as waste isolation. Although the procedure for predicting site amplifica- tion effects still has some elements of controversy, em— pirical data from past earthquakes and laboratory DETERMINE LOCAL GROUND RESPONSE 59 100.0 100.0 ——-suumes i’ABK \ —.STATION 82] D Z O U 1.” a”; x _ Site transfer furction 5‘: Es 10.0 E? 3 5 Lu a: ; E. 5 g E a w z . “<5 s E 5 - Lu 1.0 E s 5 E 2 a ”J Lu 2 > '3: 3 " o g; ; 0.1 o of n 3 Lu 2 0.01 0.01 0.1 1.0 10.0 PERIOD, m secouns . PERIOD,IN SECONDS LAS VEGAS AREA \ EXPLANATION \‘\\\i\\\\ \\ m"‘ 7 \ Contour showing depth of a" 9% 00‘ ‘59 1 f\ \\ 00 alluvium,in meters ‘ \ ‘ S ‘ ~90 \ / Dashed where position uncertain a 0 cl Seismograph stations 5 I . _.__ STATION 821 SQUIRES PARK 0 Q 0 O 0 STATION 801 0 ° 0 O C o 0 ° \ ° \ x f o \ .\H\ ”41“ ‘\ FIGURE 57.—Location of seismograph stations and thickness of alluvium in Las Vegas Valley and variation of horizontal velocity response spectra for two stations and their site transfer function. Ground motions are from nuclear explosions at the Nevada Test Site. 60 PROCEDURES FOR ESTIMATING EARTHQUAKE GROUND MOTIONS GROUND RESPONSE Station 801 2.5 1.5 1.0 EXPLANATION —_‘|'5—_ . Contour showing radial component of relative spectral velocity response in the period band 3.33-4.50 s Seismograph station THICKNESS 0F ALLUVlUM \(\ \\ _\\ x \ \ 29 \ ‘ l riot ‘ )/ Y 4,00 \ F EXPLANATION —08— . Contour showing estimated thickness of alluvium, in kilometers Seismograph station FIGURE 58.—Radial component of relative ground response in the period band 3.33—4.50 s and thickness of alluvium, Las Vegas Valley (from Murphy and Hewlett, 1975). Ground response is calculated relative to station 801. 100.0 I , , Illll I II lllll Site model Site transfer function [31 = 490 m/s I .01 = 1.8 g/cm3 _ o,=10 ‘* _ [32 = 1740 m/s pz= 2.2 g/cm3 oz=oo UPHOLE SYSTEM DOWNHOLE SYSTEM r l llllll I \\~ \ “ RATIO OF VELOCITY SPECTRA. _ Observed / // \\ ll _ Th ' - 1.0 L eiorenical-i—i‘l'fi I l l 0.1 1.0 10.0 FREQUENCY, IN HERTZ FIGURE 59,—Variation of ground motion with depth, Beatty, Nev. studies (for example, Idriss and Seed, 1968; Espinosa and Algermissen, 1972; Seed and others, 1972; Hays and King, 1973; Warrick, 1974; Campbell and Duke, 1974; Page, Boore, and Dieterich, 1975; Borcherdt and others, 1975; Seed, 1975; Joyner and Chen, 1975; Hays and others, 1979) suggest the following guidelines: 1. Spectral ratios (transfer functions) of the ground motion at sites underlain by 'uncon- solidated materials and at sites underlain by rock may be as high as 10 to 1 for some periods when low-strain ground motions are involved. However, this ratio is generally thought to decrease substantially for intense high-strain ground motions, depending on the material properties. 2. Amplification is a sensitive function of the thickness of the unconsolidated materials overlying rock; increasing thickness gen- erally lengthens the period at which amplifi- cation occurs. 3. The shorter period spectral components are usually damped out fairly rapidly by thick DETERMINE LOCAL GROUND RESPONSE soft materials. This effect tends to reduce the level of peak ground acceleration when the peak rock acceleration exceeds 0.1 g. 4. The top 30 In or so of a deposit of unconsolidated material frequently has the most criticalef- fect upon amplification and should be inves- tigated thoroughly to determine its dynamic properties. 5. Unconsolidated material below the topmost 30 m generally does not need to be investigated for nonlinear behavior unless the specific material is especially sensitive to ground mo- tion. 6. For long-duration high-strain ground motions, nonlinear effects become very important. The general effects of nonlinear behavior are to 61 decrease the amplification level (relative to what would have occurred for linear behav- ior) and to lengthen the characteristic site period. For low-strain earthquake ground motions, uncon- solidated materials can respond in a manner close to that which the theory of elastic wave propagation pre— dicts. These effects represent realistic upper bounds of ground response. Parametric curves (fig. 60) published by Environmental Research Corporation (1974b) for amplification of incident, plane SH waves (Haskell, 1960) show how the response characteristics of a low- velocity material overlying rock change as the physical properties of the material and the angle of wave inci- dence change. The curves are plotted against the di- mensionless parameter d/Bl -f where d is layer thick- A IIlI= IIII ITIT |||I TTII IIII IITI IIII Ill! 4F II {3,92 1591 {I II fl ' A erfzfiplln ; In: ‘ I— I" I‘\p=‘l|5p I"\ I‘ I'\\ _ I— I"\ I" I“ [I‘\ I‘ _ 2 A‘- Il/VZ ' 1 IN m I/\\ m I/\\ ’ \ ./ \/ \/ \/ \j \_ \_/ \% 1.5p1 J \/ \: 0 I I I I I I I L 1 | | I I I I I I I I I ' ‘ ' I 1 1 1 l l I l l | l I 1 l l I I 10 llr‘ll Il‘l'l I=] I I'fli'Il llfill II'N|II IIII IIIIIIIII IIII IIII 8—3 lIll i[€132 2.5m:l :1 I: — —Efi J¥p2=25p1f1 {I {I _ — ll| II II II II _ _ 'I :I H II I| a 6 .I II II I' I' I' I| .“I II II _ }I I I} i I ,I l'l "'. I' II| .1: 4 I I I _ — I It A! Il _ E -I\ IIZ’”I\ /\ /\ -- I”2=‘“II II II- < u. 2 ‘ E / \/ \/ \/ \/ \.= J \J \J \/ \_/ \ I; 0 I I I I . I I I II I I 4 I L; . . 1 1 I I I I I l I I I I I I I I I I I I I I E 14 IIIII IIIII IT 'I III] [III III! 'I I II II II [III IIII : —CI I _ _ _ - <5: 12 I Isz = 2 5 p1 Il "r92 — 2'5 ’01 ii fl IL I' I I II 'I Ii II I' 5' [I I II [I - ‘ II I II II II ‘ ll II II L I I. II .I .1 I 10 l 'I 'I " ll 7' 'I " " lI _ I I I I - II II I| II I. - 8 II I. I! II I. I l' I: ' ' 1 ' l I {I ‘ 6 .- I”2“'5“III II II ~ . I I I - I II I\ II-- I II‘WS‘II I I ~ 2 / \/ \/ \/ / \ J K/ \/ \_/ \J k o0111,05l llILOI I] ILSII I l2.011'12.5(Il1110.5|IIIHIl lll1.5l|l l2.011LL2.5 DIMENSIONLESS FREQUENCY FIGURE 60.—Parametric curves for amplification of SH waves. A, [32 ZZBI, i=0, elastic response. B, 82:43,, i=0°, elastic response. C, 32:63., i=0°, elastic response. D, [33:23“ i=30°, elastic response. E, [32:43“ i=30°, elastic response. F, 32:6131, i:30°, elastic response. From Environmental Research Corporation (1974b). 62 PROCEDURES FOR ESTIMATING EARTHQUAKE GROUND MOTIONS ness, ,81 is shear wave velocity of the low-velocity layer, and f is frequency. For a fixed angle of incidence, the amplification factor depends only upon the acoustic impedance contrast and not upon the actual values of shear-wave velocity and density. Curves are given for two density contrasts (pz/p1= 1.5 and 2.5), three shear-wave velocity contrasts (fig/31:2, 4, and 6), and two angles of incidence, (i=0° and 30°). To use the parametric curves, one needs values of the ratio of shear wave velocities 31/131 and thickness d of the low-velocity material. The ratio d/Bl provides the scale factor which converts the abscissa to frequency in hertz. An example showing how the site transfer function is estimated is depicted in figure 61. If the density ratio is 1.35, the velocity ratio 7.5, and the angle of incidence 30°, the amplification factor is interpolated from figure 60F as follows: AI p1‘31 _7-8 = (1-5)_(6) _ orA'=8.8. A’ (1.35) (7.5) Thus, the amplification is a factor of 8.8. The resonant frequency is found from the relation (fig. 60 F): A f=0.25 or 6-4 B1 183 so f is approximately 7 Hz for elastic response. f=0.25 10 a1 = 305 m/s I (31 = 183 m/s p1=1.5g/cm3 6.4 m a2 = 2270 m/s 82 = 1382 m/s p2 = 2.09/cm3 AMPLIFICATIUN FACTOR FREQUENCY, IN HERTZ FIGURE 61.—Example of site transfer function, elastic response. a. and 012 are P-wave velocities, 131 and B2 are S-wave velocities, and p. and p2 are densities. Amplification above 10 Hz is not normally consid- ered because high-frequency shear waves attenuate very rapidly. The site transfer function for the case of multiple low-velocity layers can also be derived by using the Haskell (1960) matrix formulation or by other numeri- cal methods. An estimate can be obtained by using figure 60 and combining the response calculated for each individual layer. Site amplification modeling is especailly needed for the case of high-strain ground-motion loads. Because of the increased number of physical parameters and the present gaps in knowledge (Kanninen and others, 1970) about inelastic behavior of solids, comprehensive parametric curves cannot be provided. The empirical procedures proposed for the 1976 Uniform Building Code (see next section) can be used to model the high- strain case and will give reasonable estimates of the effect. Also, some of the general curves (figs. 62—64) that have been published (Imbsen and Gates, 1973) permit estimates to be made for high-strain loading conditions. These curves were derived with the SHAKE program (Schnabel, Lysmer, and Seed, 1972). Values of the peak horizontal rock acceleration and the thickness of the unconsolidated material are needed when estimating amplification effects for high-strain earthquake ground motion. The accelera- tion estimates can be derived from maps such as shown in figure 42 or by empirical “deconvolution” techniques (for example, Lysmer and others, 1971; Schnabel, Seed, and Lysmer, 1972; Papadakis and others, 1974). By multiplying the peak rock acceleration by the rock re- sponse factor (fig. 62), an estimate of the elastic- response spectrum for rock is obtained. Amplification FREQUENCY, IN HERTZ 5.0 3.33 2.5 1.67 1251.0 0.67 0.5 0.33 0.25 5 I I I I I I I I I I Peak acceleration levels ROCK RESPONSE FACTOR 0 0.1 0.2 0.3 0.4 0.6 0.81.0 1.5 2.0 3.0 4.0 5.0 PERIOD, IN SECONDS FIGURE 62.——Normalized rock response spectrum, 5-percent damping (modified from Imbsen and Gates, 1973). DETERMINE LOCAL GROUND RESPONSE 63 effects relative to rock are determined by multiplying the appropriate curve of figure 63 and the estimated elastic-response spectrum for rock (fig. 62). This pro- duct gives the approximate amplification effects. The peak rock acceleration that is transmitted to the soil column has a major effect on the amplification spectra. As the peak rock acceleration increases, the amplification level decreases and the predominant period of resonance lengthens (fig. 64). The generalized parametric curves of figures 63—65 may not provide the level of precision needed for pre- dicting amplification effects at the site of interest. In this case, it might be desirable to utilize a finite- FREGUENCY, IN HERTZ 5.0 3.33 2.5 1.671.251.0 0.67 0.5 0.33 0.25 5 I l I I ,I I I I I I Peak rock acceleration = 0.2 g a: 4 — — o I- U :5 3 _ _ g 24.4-45.7 m 4:176? m _ f ”u - o aIIUVIum ,_ u a VlUm g _ 15.2-24.4 m _ E 2 of alluvium 3 D. E < 1 | I I I I I I 0 I I I 0.1 0.2 0.3 0.4 0.6 0.81.0 1.5 2.0 3.0 4.0 5.0 PERIOD, lN SECONDS FIGURE 63.——Parametric curves for high-strain amplification of SH waves and three depths of unconsolidated materials (modified from Imbsen and Gates, 1973). FREQUENCY, IN HERTZ 5.0 3.33 2.5 1.87 1251.0 0.67 0.5 0.33 0.25 5 I I I I I ‘ I I I I Alluvium thickness, 2445 m a: 4 _ _, o I— i? ”' Peak rock 5 3 '_ acceleration fl IT. 2 — $1199 — E \__/ QP‘QQ ‘2‘ 1&333“ — 0 I I l I I I l I I I 0.1 0.2 0.3 0.4 0.6 0.8 1.0 1.5 2.0 3.0 4.0 5.0 PERIOD,IN SECONDS FIGURE 64.—Effect of peak acceleration level on amplification (modified from Imbsen and Gates, 1973). element program such as SHAKE (Schnabel, Lysmer, and Seed, 1972) or QUAD—4 (Idriss and others, 1973), or the technique proposed by Joyner (1975) to model the site in more detail. The degree of accuracy to which the physical properties of the rock and unconsolidated materials at the site are known is the primary consid- eration before deciding to use sophisticated calcula- tional procedures. Several new earthquake-design provisions, includ- ing a procedure for estimating the characteristic site period, were proposed for incorporation in the 1976 Uniform Building Code (Freeman, 1975). These pro- visions will be discussed below. SUMMARY OF UNIFORM BUILDING CODE PROCEDURES The most widely used standard for earthquake- resistant design is the Structural Engineers Associa- tion of California (SEAOC) Code. The SEAOC Code, which was incorporated into the 1973 Uniform Build- ing Code, contains the following commentary about aseismic design: Basically, the problem is that the entire phenomenon, from the earthquake ground motion to the realistic response of structures to this ground motion, is very complex. Codes, of necessity, are gen- eralized simplifications. Complex mathematical analyses have been made on simple and idealized structures subjected to past earth- quake ground motions. These have been helpful in improving our understanding of the phenomenon. However, for purposes of design of the vast majority of structures, it is necessary to reduce this com- plex, dynamic problem to one of equivalent static lateral forces. These can be related to the dynamic characteristics of the structure. They provide the basic code criteria, applied with stresses within the elastic limit. However, in applying these simplified concepts, the structural engineer must do this with sound judgment that can only be developed with experience, observation, and study of the earth- quake phenomenon. He must be especially aware of the nature of the response of the particular structure under design and he must evalu- ate the capabilities of that structure to perform satisfactorily beyond the elastic-code-stipulated stresses. The SEAOC Code is a minimum standard to assure public safety. The requirements are intended to safeguard against major failures and loss of life. The. aim of the code is to provide structures that will: 1. resist minor earthquakes with damage; 2. resist moderate earthquakes without structural damage, but with some nonstructural dam- age; 3. resist major earthquakes without collapse, but with some structural and nonstructural damage. The basic formula used in the 1973 Uniform Building Code is V = ZKCW where V is base shear or total lateral force to be resisted 64 PROCEDURES FOR ESTIMATING EARTHQUAKE GROUND MOTIONS at the base of the structure, Z is zone factor, based on the Algermissen (1969) US. seismic risk map (fig. 65) having four seis- mic risk zones. The numerical values of Z for the four zones are: Z: 1 for zone 3; Z :05 for zone 2; Z=0.25 for zone 1; and Z=O for zone 0. K is an arbitary factor that, in effect, is a safety factor adjustment based on an arbitrary classification of the type of construction. It rec- ognizes that different degrees of hazard against collapse are inherent in different types of con- struction; K has no relation to the actual forces expected. C is a coefficient related to the period of the struc- ture. A plot of C against period represents a re- sponse spectrum and roughly parallels that de- rived from the 1940 Imperial Valley, Calif. earthquake accelerogram recorded in El Centro, Calif, and W is the weight of the structure. Several new design provisions were proposed for the 1976 Uniform Building Code (Freeman, 1975; Nosse, 115° 110° 105° 100° 1975; Seed, 1975; and Donovan, 1975). The basic con- sideration is that, in general, structures shall be de- signed and constructed to resist minimum total lateral seismic forces assumed to act nonconcurrently in the direction of each of the main axes of the structure in accordance with the formula V = ZIKCSW . In this formula, Z is a zoning factor based on the pre- liminary design regionalization map (fig. 66) prepared by the Applied Technology Council (1976). K is a nu- merical factor determined by the type of framings, 0.067 V T where T is the period of the building, and S is the soil-structure interaction factor or resonance coeffi- cient, which is a function of the building period and the characteristic site period T, . I is the importance factor for the building and W is the total dead load or weight of the building. The values of Z, the seismic zoning factor, are de- fined as equal to 1 for locations in zone 4, 34 for loca- C: 75° 70 95° 90° 85° so 45° 40° 30° EXPLANATION ZONE O—No damage. ZONE l—Minor damage; distant earthquakes may cause damage to structures with iundamental periods greater than 1.0 second; corresponds to intensities V and VI of the Modified MercaIIi scale ZONE 2—Moderate damage; corresponds to intensity VII of the Modified Mercalli scale. 25° ZONE 3—Major damage; corresponds to intensity VIII and higher of the Modified Mercalli scale. This map is based on the known distribution of damaging earthquakes and the Modified Mercalli scale intensities associated with these earthquakes. I MEXICO o 500 KILOMETERS I_I_I_l_l_| 115° 110° 105° 100° l 95° 90° 85" ° 80 75 FIGURE 65.—United States seismic risk zones. Based on Modified Mercalli intensity scale and the distribution of damaging earthquakes (from Algermissen, 1969). The map of seismic risk zones in the 1979 Uniform Building Code is based on this map. DETERMINE LOCAL GROUND RESPONSE 65 EFFECTIVE PEAK ACCELERATION 0.44 g » 0.22 g / CONTOUR 4 0F MEXICO 25° \ 3 GULF ‘ 2 0.11 g '7 1 0.03 I O 500 KILOMETERS 1 l 115° 110° 105 100° 95° 90° 85° 80° 75° FIGURE 66.—Pre1iminary design regionalization proposed for 1976 Uniform Building Code (from Applied Technology Council, 1976). tions in zone 3, 3/3 for locations in zone 2, and 3/,“ for locations in zone 1. Values of S, the soil-structure interaction factor, range from 1 to 1.5 (fig. 67.) Both theory and empirical data suggest the S should have its lowest values when the ratio of the building period T to the characteristic site period Ts is either very low or very high. The maximum value occurs when T/TS =1. Determination of the characteristic site period T, is an important new proposal. Studies of ground-motion characteristics and building damage from past earth- quakes have shown that the characteristic period at which maximum damaging effects occur is approxi— mately equal to the fundamental period of the soil de- posit overlying rocklike formations. A rocklike forma— tion may be considered as any earth or rock material in which the shear-wave velocity at small strains (0.0001 percent) is about 760 m/s or greater. Shear—wave veloc- ities of soils approach 760 m/s at depths of about 152 m; therefore, it is unnecessary to consider greater depths when determining the value of Ts. Values of T, will vary from a minimum value of 0.5 s for shallow, stiff soil deposits to a maximum value of 2.5 s for deep, soft soil deposits (Seed, 1975). An equivalent single-layer method was proposed for calculating the characteristic site period. The basic re- lation is Ts' : 2.1.1. . R ,3 where H is the depth of soil over bedrock, B is the average shear—wave velocity of the soil layer as meas- ured under low-strain conditions in the field, andR is a correction factor to allow for the reduction in shear- wave velocity when the soil is excited by high-strain ground motion during an earthquake. It is necessary to specify the level of base rock excitation in order to as- sign values to R. Values of R have been established on the basis of the “effective peak acceleration” obtained from the design regionalization map (fig. 66) as follows: 1. zone 1: A magnitude—6 earthquake producing a peak effective acceleration of 0.1 g corre- sponds to a value of 0.9 for R. 2. zone 2: A magnitude-6 earthquake producing a peak effective acceleration of 0.2 g corre- sponds to a value of 0.8 for R. 3. zone 3: A magnitude-7 earthquake producing a peak effective acceleration of 0.3 g corre- 66 PROCEDURES FOR ESTIMATING EARTHQUAKE GROUND MOTIONS 1.6 — 1.5 '- SOI L-STRUCTURE INTERACTION FACTOR ' 0 0.5 1.0 1.5 2.0 2.5 BUILDING PERIOD SITE PERIOD FIGURE 67.—-Soil-structure interaction factor proposed for 1976 Uni- form Building Code (from Seed, 1975). sponds to a value of 0.67 for R. 4. zone 4: A magnitude-7 earthquake producing a peak effective acceleration of 0.4 g corre- sponds to a value of 0.67 for R. As an example, consider a 12.2-m layer of sand with a shear wave velocity of 274 m/s overlying rock in Los Angeles (zone 4). For this example, 2 (4) (40) ‘° (0.67) (900) This value is less than the minimum allowable value of 0.5 s, so the value of T, used in seismic design for this example would be 0.5 second. For multilayered soil profiles, the equivalent single- layer procedure can be used to compute an approxi- mate value for T. The basic relation for T,, 111 R B ’ now let H be the total thickness of the unconsolidated material overlying bedrock and B the mean shear-wave velocity, weighted in proportion to the velocities and thicknesses of the individual layers as follows: = 0.27 seconds T.- where B,- and H ,- are the shear-wave velocity and thick- ness of each individual layer. The importance factor I has also been proposed for incorporation in the Uniform Building Code formula for base shear to allow higher force levels to be as- signed to structures housing certain facilities. The val- ues of I range from 1.0 to 1.5, with the highest value being assigned to essential facilities such as hospitals, communiCation centers, and fire stations where the ac- ceptable level of risk from earthquakes is to be re- duced. The overall effect of all the new provisions is to pro- vide for an increase in the design base shear. The details about structural design in terms of the Uniform Building Codes are beyond the scope of this paper. References such as Degenkolb, Dean, and Wyl- lie (1971), Freeman (1975), Seed (1975), and Donovan (1975) provide information on this subject for the in- terested reader. DEFINE UNCERTAINTIES OF THE GROUND-MOTION DESIGN VALUES Specification of the uncertainty model for the ground-motion design values is a complex task. The model depends upon the seismotectonic province where the earthquake occurs and the physical parameters controlling the source, path, and local ground-response effects (table 23). The complexity of the problem arises from two factors: (1) the statistical distribution of many of the physical paramenters is either unknown or poorly known and (2) the precise way to combine the uncertainties of the individual physical parameters of the system is not clear, even if the statistical distribu- tion for each parameter were well known. An example of the second factor is illustrated by the problem of combining the uncertainty in magnitude and seismic moment, two parameters that affect the low-frequency ground motion characteristics and are related to the length of fault rupture. At the present time, it is impossible to specify the exact location and magnitude of the earthquakes that will affect a site during the life of a structure. This diffi- culty is a consequence of the short incomplete seismic- ity record, the lack of regional seismicity networks to define current seismicity patterns, and the lack of adequate geologic data to define the earthquake poten- tial of a 3-dimensional volume of rock. An example will illustrate the problem. Yucca Flat and Pahute Mesa, two areas on the Nevada Test Site, are probably the UNCERTAINTIES OF THE GROUND-MOTION DESIGN VALUES TABLE 23.—Uncertaintiesrin physical parameters that affect ground 67 TABLE 23.—Uncertainties in physical parameters that affect ground motion motion—Continued Physical Effect on Uncertainty and Physical Effect on Uncertainty and parameter ground motion functional dependence parameter ground motion functional dependence Seismicity Parameters Local Ground Response—Continued Seismic ________ Zone controls location Not known. Function of Ground-motion data source zones of earthquakes. seism1c1ty record, . sample for eaph rock eologic, and tectonic and sell claSSIfica- history. tion IS small. Recurrence ____Defines frequency Average [7:045 in Soil thicknessuAffects domlnant fre— Not well defined. De- rates (b) of occurrences. eastern US. where log N=a—bI; 0'=f(N). Not known. Function of completeness and length of seismicity record and geologic data on fault rupture. Upper-bound __Establishes ground- magnitude motion design levels. and geometry. quency, duration, pseudo-ellipticity of wave particle motion, and damping. Strain level of Determines if ground pends on geophysical, geologic, borehole, and ground-motion data. Not well defined be- Source Parameters Epicenter ______ Establishes location of Best location accuracy design earthquake. is 1 km; worst is 50 km. Function of regional velocity model and in- strument locations. Best location accuracy is 2 km; worst is 50 km. Function of re- gional velocity model and instrument locations. Best accuracy is 0.1 Focal depth _ __ _Affects partition of body-and surface-wave energy Magnitude ____Affects low frequencies (mh,M,,,M,) and ground-motion unit; worst is > 1 scaling. unit. Function of in- struments and regional calibration. Seismic ________ Affects low frequencies, log M,,~3/2 M, until moment (M0) especially for great M02102“ dyne-cm. earthquakes. M,,=21.9+3 log L with scatter a factor of 2. Function of instrument dynamic range. AU has a log-normal dis- tribution. Earthquakes exhibit a constant average stress drop of about 10 bars 20': one order of magnitude. Function of moment determi- nation. M,,=1.235 +1243 logL; Stress drop ____Affects hiigh frequen- (AU) cies an peak acceleration. Fault length --Affects magnitude and (L) moment; duration. Epicentral ____Affects site accelera- intensity (1,) tion (AH and A.~). _ o=2.19 logAy = 0.28 [MM — 0.40; 0:2.53 (worldwide data) Path Parameters Rate of atten-"Establishes peak ground- Not well defined because uation of motion values at site of limitations on data seismic and frequency-de- sample. a for peak ac- energy with pendent signature. celeration vs distance distance. relation is 2.01 for worldwide data and 1.62 for San Fernando earthquake. a for peak velocit vs distance is 1.5 or moderate U.S. earthquakes. Local Ground Response Soil/rock ______ Affects amplitude of Not well defined. Phys- acoustic im- ground motion. ical properties depend pedance (pB) on geop ysical and lab- contrasts. oratory measurements. cause of limitations of the ground-motion data sample, especially near the source. Repeatable with 0': 1.30 for nuclear explosions and 1.50 for earth- quake aftershocks. input ground response is linear motlon. or nonllnear. Site transfer __Determines relative functlon. ground response etween two Sites. only areas in the United States that come close to fulfil- ling all three requirements listed above. More than 900 man-years of geologic mapping and many deep drill holes have defined the 3-dimensional structure reason- ably well. A regional seismicity network has defined the current seismicity patterns. Fairly good correla- tions have been made between current seismicity and specific active faults. Even with this information, as- signing a magnitude to a given active fault on the basis of the assumption that 50-percent of its length is avail- able to rupture gives a magnitude value with a stan- dard deviation of almost 1 unit (see Mark, 1977). A general model based on the use of mean values for a "lumped parameter” wave-progagation system pro- vides some insight. The basic relation is given by Szg‘ystpelmz (1 + 131— ) (S2source ,+ S2path + SZsite) where N is the size of the data sample, Ssource is the variance of the source, S path is the variance of the transmission path. (Note: variance = S = log a where o- is the geometrical standard deviation.) The values of S can be approximated from values of 0- reported in the literature. The distribution is assumed to be log-normal in each case, a reasonable assumption. The studies by N. M. Newmark Consulting Engineer- ing Services (1973) and J. A. Blume and Associates, Engineers (1973) provide a value of 0.30 for SSW“. (a = 1.35). Donovan ( 1973) studied a worldwide data sample of 515 peak acceleration values to derive a relation for peak acceleration and distance. The variance for this "general” transmission path is 0.70 (a = 2.01). The studies of ground response by Murphy, Weaver, and Davis (1971) and Murphy, Lynch, and O’Brien (1971) 68 PROCEDURES FOR ESTIMATING EARTHQUAKE GROUND MOTIONS show that the mean site amplification is repeatable for low—strain ground motions with a variance that ranges from 0.26 to 0.41 (corresponding values of 0' are 1.30 and 1.50). A reasonable value of the “general” site variance is 0.41. Combining these numbers after ad- justing for the size of the data sample gives $210.,l = 0.095 + 0.491 + 0.178 8T4... = 0.87407 and sys em a = log’1 0.87407 = 2.40 Thus, an estimate of the geometrical standard devia- tion for mean ground motion estimates is about 2.40. However, one should remember that upper-bound val- ues, not mean values, are used to define many ground-motion estimates. This practice suggests that the ground-motion estimates in some cases (for exam- ple, nuclear powerplants) may be at the one or two— standard deviation level relative to the mean value expected to occur at the site. It is clear that the uncertainty in the seismic attenu- ation relation contributes most to the total uncer- tainty. One significant way to reduce the uncertainty (or the conservatism) in the ground-motion estimates is to “calibrate” the region’s attenuation characteris- tics. As an example, consider the situation where the San Fernando Valley, California attenuation relation can be applied instead of the “general” relation. Dono- van (1973) showed that the value of variance is 0.48 (o- = 1.62) for the San Fernando Valley. Substituting 0.48 instead of 0.70 and retaining the other values gives a o- of 2.03 for the total system; therefore, the “calibrated” attenuation function provides a significant reduction in total uncertainty. SEISMIC DESIGN TRENDS FOR THE FUTURE There is little doubt that empirical procedures currently used for defining earthquake ground motion will be refined and extended in the future. However, greatly improved capability for specifying earthquake ground motion will require a significantly improved data base. Regional seismicity and strong-motion ac— celerograph networks must be expanded. Better geologic and geophysical data and analysis are needed to define the earthquake potential and upper-bound magnitude in different geographical regions and to es- tablish the dynamics of faulting and recurrence inter- vals for specific faults. Knowledge of the origin of in- traplate earthquakes, which seem to have different causes than the earthquakes that occur along plate boundaries, is needed in order to assess more accu- rately the earthquake potential of seismically quiet re- gions like the Eastern United States. The quality of the data base is one of the most impor- tant factors leading to the capability for precise specification of earthquake ground motion. The “ideal” data base should contain complete information about the site and the region surrounding it, including a well—defined statistical distribution for each parameter and parametric relation used to make predictions of ground motion at the site. The basic components are: 1. Seismicity parameters A complete record of all historic earthquakes; Information about the source parameters (epi— center, focal depth, source mechanism and dimensions, magnitude, stress drop, effective stress, seismic moment, and rupture velocity) of each historic earthquake; Definition of the current seismicity patterns; Recurrence relations for the region and for well-defined seismic source zones in the re- gion. 2. Seismotectonic features Maps showing seismotectonic provinces and ca- pable faults; Information about the earthquake generating potential of each seismotectonic province, in- cluding: information about the geometry, amount, and sense of movement, the tem- poral history of each fault, and the correla- tion with historic instrumental earthquake epicenters; Correlation of historic earthquakes with tec- tonic models to estimate the upper-bound magnitude of the earthquake likely to be as- sociated with each tectonic feature. 3. Seismic attenuation function Isoseismal maps of significant historic earth— quakes occurring in the region; The uncertainty of Modified Mercalli intensity distance-scaling relations; Peak ground-motion distance-scaling relations for the region; Frequency—dependent distance-scaling rela- tions for the region. 4. Characteristics of ground shaking Isoseismal maps of significant historic earth- quakes that have affected the site; Ensembles of strong ground-motion records adequate for calibrating the near field, the regional seismic-wave transmission charac- teristics, and the local ground response for a wide range of earthquake source mecha— nisms. 5. Earthquake spectra Ensembles of spectra (Fourier, power spectral density, and response) adequate for calibrat- REFERENCES CITED 69 ing the near field, the transmission path, and the local ground response for a wide range of earthquake source mechanisms. 6. Local ground response Strong ground-motion records at surface and subsurface locations for a wide range of strain levels; Seismic-wave transmission characteristics of a wide range of unconsolidated materials over- lying rock for a wide range of strain levels; Information on the static and dynamic prop- erties of the near-surface site materials, in- cluding: seismic shear-wave velocities, bulk densities, and water content. The faulting mechanism, more than any other geologic parameter, requires a great deal of additional research. The direction, length, and type of faulting during an earthquake greatly affect the near- and far- field radiation patterns'of the body and surface waves generated by the earthquake. As progress is made in understanding faulting, important questions can be answered, such as: 1. What are the important differences in the near- and far-field seismic-radiation patterns for different fault systems? 2. What is the best way to characterize the near— and far-field seismic radiation patterns in areas of the United States where surface faulting is uncommon (for example, New Madrid, Missouri and Charleston, South Carolina areas)? Although the present empirically based procedures will remain in use for some time, improved procedures will evolve as advances in physical understanding and capability to model numerically occur. Improved pro- cedures will incorporate the continually increasing knowledge in seismology and will improve the preci- sion of earthquake ground-motion estimates. Some of the new approaches that are likely include: 1. Use of seismic moment as a measure of source strength instead of or in addition to mag- nitude, 2. Use of constant stress—drop source spectra scaled as a function of seismic moment in- stead of magnitude, 3. Interpretation of peak ground—motion parame- ters in terms of source spectra, 4. Interpretation of duration of strong ground mo— tion in terms of length of fault rupture and rupture velocity, 5. Development of spectral values to correlate with specific values of the Modified Mercalli intensity scale, 6. Development of frequency-dependent seismic attenuation laws for many different geo- graphic regions of the United States. Earthquake-resistant design is fortunate to be a dynamic field in which many advances in basic knowledge are being made. REFERENCES CITED Aki, Keiiti, 1972, Scaling law of earthquake source time function: Roy, Astron. Soc, Gebphys. Jour., v. 31, p. 3—25. Albee, Arden, and Smith, J. L., 1966, Earthquake characteristics and fault activity in southern California, in Lung, R., and Proctor, R., eds, special publication, Engineering geology in southern California: Eng. Geologists Assoc, p. 9—33. Algermissen, S. 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Thatcher, Wayne, Hileman, J. A., and Hanks, T. C., 1975, Seismic slip distribution along the San Jacinto fault zone, southern California, and its implications: Geol. Soc. America Bull., v. 86, p. 1140—1146. Tobin, D. G., and Sykes, L. R., 1968, Seismicity and tectonics of the northeast Pacific Ocean: Jour. Geophys. Research, V. 73, p. 3821— 3845. Townley, S. D., and Allen, M. W., 1939, Descriptive catalog of earth- quakes of the Pacific coast of the United States: Seismol. Soc. America Bull., v. 29, p. 1—20. Trifunac, M. D., 1970, Low frequency digitization errors and a new method for zero baseline correction of strong motion accelero- grams: Pasadena, Calif, California Inst. Technology, Earth- quake Eng. Research Lab. Rept. EERL 70—07, 30 p. 1971, Response envelope spectrum and interpretation of strong earthquake ground motion: Seismol. Soc. America Bull., v. 61, p. 343—356. 1972a, Stress estimates for San Fernando, California, earth- quake of February 9, 1971: Main event and thirteen aftershocks: Seismol. Soc. America Bull., v. 62,0. 721—770. 1972b, Tectonic stress and the source mechanism of the Impe- rial Valley, California, earthquake of 1940: Seismol. Soc. America Bull., v. 62, p. 1283-1302. 1973a, Scattering of plane SH waves by a semi-cylindrical canyon: Intl. Jour. Earthquake Eng. Struc. Dynamics, v. 1, p. 267—282. 1973b, Analysis of strong earthquake ground motion and pre- diction of response spectra: Intl. Jour. Earthquake Eng. Struc. Dynamics, v. 2, p. 59—69. 1976a, Preliminary analysis of the peaks of strong earthquake ground motion-dependence of peaks on earthquake magnitude, epicentral distance, and recording site condition: Seismol. Soc. America Bull., v. 66, p. 189—220. 1976b, Preliminary empirical model for scaling Fourier ampli- tude spectra of strong ground acceleration in terms of earthquake magnitude, source-to—station distance, and recording site condition: Seismol. Soc. America Bull., v. 66, p. 1343— 1374. Trifunac, M. D., and Brady, A. G., 1975a, Correlations of peak accel- eration, velocity, and displacement with earthquake magnitude, and site condition: Int]. J our. Earthquake Eng. Struc. Dynamics, v. 4, p. 455—471. 1975b, A study on the duration of strong earthquake ground motion: Seismol. Soc. America Bull., v. 65, p. 581—626. 1975c, 0n the correlation of seismic intensity scales with the peaks of recorded ground motion: Seismol. Soc. America Bull., v. 65, p. 139—162. Trifunac, M. D., waadia, F. E., and Brady, A. G., 1971, High fre- quency errors and instrument corrections of strong-motion ac- celerograms: Pasadena, Calif, California Inst. Technology, Earthquake Eng. Research Lab. Rept. EERL 71—05, 40 p. Trifunac, M. D., and Westermo, B. D., 1977, Dependence of the dura- tion of strong earthquake ground motion on magnitude, epicen- tral distance, geologic conditions at the recording station, and frequency of motion: Southern California Univ., Dept. of Civil Eng, rept. CE76—02, 64 p. Uniform Building Code, 1973, International Conference of Building Officials, Whittier, California (updated every 3 years) 704 p. 1976, International Conference of Building Officials, Whittier, California (updated every 3 years) 728 p. US. Atomic Energy Commission, 1971, Reactor site criteria: Appen- dix A: seismic and geologic siting criteria for nuclear power plant sites: Federal Register, v. 36, p. 22601—22626. 1973a, Reactor site criteria, seismic and geologic siting criteria (amendments): Federal Register, v. 38, no. 318, 2 p. 1973b, Design response spectra for seismic design of nuclear power plants, (revision), Regulatory Guide 1.60, 8 p. US. 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Atomic Energy Comm., 96 p. West, L. R., and Christie, R. K., 1971, Observed ground motion data, Cannikin event: Environmental Research Corp. Rept. NVO— 1163—230 to US. Atomic Energy Comm., 26 p. REFERENCES CITED 77 Wiggins, J. H., and Moran, D. F., 1971, Earthquake safety in the city of Long Beach based on the concept of balanced risk: J. H. Wig- gins Company rept. to City of Long Beach, 129 p. Wilson, J. T., 1973, Mantle plumes and plate motions: Tec- tonophysics, v. 19, p. 149— 164. Wong, H. L., Trifunac, M. D., and Westermo, B., 1977, Effects of surface and subsurface irregularities on the amplitudes of monochromatic waves: Seismol. Soc. America Bull., v. 67, p. 353—368. Woolard, G. P., 1958, Areas of tectonic activity in the United States as indicated by earthquake epicenters, transactions: Am. Geophys. Union Trans, v. 39, p. 1135—1150. 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Part B. Meteorology and Hydrology in Big Thompson River and Cache la Poudre River Basins ByJERALD F. MCCAIN of the US. GEOLOGICAL SURVEY and LEE R. HOXIT, ROBERT A. MADDOX, CHARLES F. CHAPPELL, and FERNANDO CARACENA of the NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION Geologic and Geomorphic Effects in the Big Thompson Canyon Area, Larimer County By RALPH R. SHROBA, PAUL w. SCHMIDT, ELEANOR J. CROSBY, and WALLACE R. HANSEN of the US. GEOLOGICAL SURVEY With a section on DAMAGE CAUSED BY GEOLOGIC PROCESSES DURING FLOOD PRODUCING STORMS ByJAMES M. SOULE of the COLORADO GEOLOGICAL SURVEY GEOLOGICAL SURVEY PROFESSIONAL PAPER 1115 Report prepared jointly by the U. S. Geological Survey and the National Oceanic and Atmospheric Administration Cooperating organization: Colorado Geological Survey UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1979 UNITED STATES DEPARTMENT OF THE INTERIOR CECIL D. ANDRUS, Secretary GEOLOGICAL SURVEY H. William Menard, Director Library of Congress Cataloging in Publication Data Main entry under title: Storm and flood of July 31—August 1, 1976 in the Big Thompson River and Cache la Poudre River Basins, Larimer and Weld Counties, Colorado. (Geological Survey Professional Paper 1115) Includes bibliographical references. CONTENTS: McCain, J. F., et al. Meteorology and hydrology in Big Thompson River and Cache 1a Poudre River Basins.—Shroba, R. R., et al. Geologic and geomorphic effects in the Big Thompson Canyon area, Larimer County; with a section on Damage caused by geologic processes during flood producing storms, by J. M. Soule. 1. Big Thompson River watershed, Cola—Storms, 1976. 2. Cache 1a Poudre River watershed—Storms, 1976. 3. Big Thompson River watershed, Colo.—Floods. 4. Cache la Poudre River watershed—Floods. I. McCain, Jerald F. II. Shroba, R. R. 111. United States Geological Survey. IV, United States National Oceanic and Atmospheric Administration. V. Colorado. Geological Survey. V1. Series: United States Geological Survey Professional Paper 1115. QC959.U6384 551.4'8 79—607003 For sale by the Superintendent of Documents, US. Government Printing Office Washington, DC. 20402 Stock Number 024—001—03223—6 GLOSSARY [References in “Glossary" are listed under “Selected References” in Part B] Aggradation. The deposition of sediment by a stream of water. Alluvium. Sediment including clay, silt, sand, and gravel in transit and(or) deposited by a stream. Excludes detritus deposited in standing water such as lakes or oceans. Altimeter setting. The pressure required to make an altimeter indicate zero altitude at an ele- vation of 10 feet above mean sea level. Cirrus anvil. High clouds which spread outward from the tops of thunderstorms. Colluvium. A deposit of unconsolidated detritus or earthy material that has been carried downslope chiefly by gravity. as opposed to running water. Generally is heterogeneous, poor- ly sorted, and poorly bedded. Includes but is not limited to talus, soil creep, landslides, avalanche deposits, and sheetwash. Colorado Piedmont. A section of the Great Plains. The part of the Great Plains lying between the Southern Rocky Mountains and the High Plains. In the Big Thompson region it is the area east of the hogback belt at the foot of the Front Range. The name “High Plains” is sometimes used erroneously for this section. Convection. Vertical motions and mixing resulting when the atmosphere becomes thermody- namically unstable. Cubic feet per second (cfs or ft3/s). A rate of discharge. One cubic foot per second is equal to the discharge of a stream of rectangular cross section 1 foot wide and 1 foot deep flowing at an average velocity of 1 foot per second. dBZ. A measure of the relative power (in decibels) of returned energy to transmitted energy. Debris avalanche. In the Big Thompson Canyon area, term is applied to a very heterogeneous mixture of water-saturated bouldery debris flowing very rapidly down a long, narrow steep channel and leaving behind a conspicuous linear scar on the mountainside. Similar to a debris flow but moving at a higher velocity down a steeper gradient (fig. A). FIGURE A.—Debris avalanche deposit and scar. iv GLOSSARY Debris fan. A fanlike or conelike subaerial accumulation of sand, gravel, cobbles, boulders, and more or less organic trash deposited where the velocity of a stream is abruptly checked by a change of gradient, as at the mouth of a gulch. Deposit is generally poorly sorted and poorly stratified. Generally, a product of torrential runoff. The fanlike form results from the shifting of the channel as the stream blocks and diverts itself repeatedly with its own debris (fig. B). FIGURE B.—Debris fan. Debris flow. A very heterogeneous mixture of water-saturated debris flowing slowly to very rapidly down a ravine and discharging onto a debris fan. In the Big Thompson Canyon area, many debris flows contained abundant woody trash, such as logs and brush. Some debris flows evidently discharged directly into the Big Thompson River and were swept away by the flood. Similar to a debris avalanche but flowing down a flatter gradient at lesser velocity. Debris slide. The most common type of landslide set off by the Big Thompson storm. In the Big Thompson area, a moist stony heterogeneous landslide that moved downward and out- ward without backward rotation. Motion may have been slow to rapid. Mostly in colluvium. Degradation. As applied to streams, synonymous with erosion. In a more general geomorphic sense, the gradual lowering of the landscape by weathering and erosive processes. Detritus. Any material worn or broken from rocks by mechanical means. The composition and dimensions are extremely variable (Stokes and Varnes, 1955, p. 37). Dewpoint (or dew-point temperature). The temperature to which a parcel of air must be cooled at constant pressure and constant water-vapor content in order for saturation to occur. Discharge. The quantity of fluid mixture including dissolved and suspended particles passing a point during a given period of time. Drainage area. The area, measured in a horizontal plane, which is enclosed by a topographic divide. Echoes. In radar terminology, a general term for the appearance on a radar indicator of the electromagnetic energy returned from a target. Entrainment. The mixing of environmental air into a preexisting cloud parcel. Fault. A fracture in the Earth’s crust along the sides of which there has been movement par- allel to the fracture plane. Flood. Any abnormally high streamflow that overtops natural or artificial banks of a stream. Flood plain. The nearly flat ground bordering a river and occupied by the river at flood stage, built up from sediment deposited when the river overtops its banks and spreads outside its low-water channel. Floodway. The channel of a river or stream and those parts of the flood plains adjoining the channel, which are reasonably required to carry and discharge the floodwater (Erbe and Flores, 1957, p. 443, quoted in Langbein and Iseri, 1960, p. 1 1). Usually applied to the part of the flood plain reserved or zoned to accommodate expectable flooding. Front. Boundary separating two different air masses. Gage height. The water-surface elevation referred to some arbitrary gage datum. Gaging station. A particular site on a stream, canal, lake, or reservoir where systematic obser- vations of gage height or discharge are obtained. Gneiss. A foliated metamorphic rock having alternate layers of visibly dissimilar minerals, especially feldspar-rich layers alternating with mica-rich layers. Very common in Big Thomp- son Canyon. Gradient. As applied to streams, the inclination of the bed, usually expressed as a percentage, or feet per mile. GLOSSARY Granite. A visibly granular igneous rock of interlocking texture composed essentially of alka- lic feldspar and quartz, commonly with a small percentage of mica and hornblende. Very common in upper reaches of Big Thompson Canyon. Granodiorite. A visibly granular igneous rock of interlocking texture similar to granite in gen- eral appearance but with soda-lime feldspar predominant over alkalic feldspar in a ratio of from 2:1 to 7:1 and with hornblende as the common mafic accessory mineral. Grus. An accumulation of waste consisting of angular, coarse-grained fragments resulting from the granular disintegration of crystalline rocks, especially granite, generally in an arid or semiarid region (Gary and others, 1972, p. 317). Hydrograph. A graph showing stage, discharge, velocity, or other property of water with re- spect to time. Inversion (Temperature inversion). A layer in the atmosphere where the temperature increases with height. Isotherm. A line of equal or constant temperature. Knot (kt). A rate of speed of 1 nautical mile per hour, equal to 1.105 miles per hour. Commonly used to express wind speed. Landslide. The downward and outward movement by falling and (or) sliding or flowing of slope-forming materials composed of natural rock, soils, artificial fills, or combinations of these materials. (See Varnes, 1958, p. 20; see also fig. C). FIGURE C.—Landslide types common along the east slope of the Front Range. From left to right: I, rockfall moves mostly by free fall, bounding, and rolling; II, slump by rotational slippage on concave-upward shear surfaces; III, debris slide by complex internal adjustments of highly deformed, sheared slide mass; IV, earthflow by displacements and velocities similar to those of viscuous fluids (Varnes, 1958, pl. 1). Types A and C were prevalent in the Big Thompson Canyon area. Small-scale slumping, type B, took place along riverbanks east of the mountains. Illustra- tions by Natalie J. Miller, from Nilsen (1972). Level of free convection. The level at which a parcel of air lifted dry adiabatically until satu- rated, and moist adiabatically thereafter, would first become warmer than its surroundings. Lifted condensation level. The level at which a parcel of moist air lifted adiabatically would become saturated. Lifted index. An index based on the difference (in °C) between the 500-millibar (mb) environ- mental temperature and the temperature of a parcel of air lifted adiabatically from or near the surface. Mass wasting. A general term for the dislodgement and downslope transport of soil and rock material under the direct application of gravitational body stresses. In contrast to other ero- sion processes, the debris removed by mass wasting processes is not carried within, on, or under another medium possessing contrasting properties. The mass strength properties of the material being transported depend on the interaction of the soil and rock particles with each other. It includes slow displacements such as creep and solifluction and rapid movements, such as earthflows, rockslides, avalanches, and falls (Gary and others, 1972, p.434). Metamorphic rock. Rock changed materially in composition or appearance, after consolida- tion, by heat, pressure, or infilitrations at some depth in the Earth's crust below the surface zone of weathering. GLOSSARY Migmatite. An intimate mixture of granitelike igneous rock and a metamorphic host rock (schist or gneiss) in which the mixture is on a small scale but is sufficiently coarse to be easily recognized by eye (Stokes and Varnes, 1955, p. 92). Common in Big Thompson Canyon. Mean sea level (msl). The average height of the surface of the sea for all stages of the tide over a 19-year period. Millibars. A pressure unit equivalent to 1,000 dynes per square centimeter, convenient for reporting atmospheric pressure. Mixing ratio. The ratio of the mass of water vapor to the mass of dry air. Peak discharge attenuation. The reduction in peak discharge of a stream along the direction of flow. Peak stage. The maximum height of a water surface above an established datum plane; same as peak gage height. Pegmatite. An igneous rock of deep-seated origin having a very coarse average grain size and an interlocking texture, composed of predominant feldspar and quartz, subordinate mica, and various accessory minerals. Percent slope. The vertical rise in slope per horizontal distance expressed as a percentage. Thus, a 10-percent slope rises 10 feet in a distance of 100 feet. Point bar. One of a series of low, arcuate ridges of sand and gravel developed on the inside of a growing meander by the slow addition of individual accretions accompanying migration of the channel toward the outer bank (Gary and others, 1972, p. 552). Precipitable water. The amount of water contained in an atmospheric column if all the vapor between two levels (usually the surface and 500 mb) was condensed. Radiosonde. A balloon-borne instrument package for measuring and transmitting meteoro- logical data. Rawinsonde. Meteorological data-collection system including a radiosonde and reflectors for measuring winds by radar. Recurrence interval. As applied to flood events, recurrence interval is the average number of years within which a given flood peak will be exceeded once. Rockfall. Rock material that plummets, bounds, or rolls down a precipitous slope. Rapid to extremely rapid movement. (See Varnes, 1958, fig. 6, p. 22.) Runoff. That part of precipitation which appears in surface streams. Schist. A visibly crystalline foliated metamorphic rock composed chiefly of platy mineral grains, such as mica, oriented so that the rock tends to split into layers or slabs. Very com- mon in Big Thompson Canyon. Sediment. Fragmental material that originated from weathering of rocks and is transported by, suspended in, or deposited by water or is accumulated in beds by other natural agencies. Sheetflood. A broad or nearly continuous cover of floodwater flowing sheetlike down a slope, as opposed to water concentrated in rills or rivulets. A result of intense but short-duration rainfall. Sheetwash. Sediment picked up and redeposited by sheetflooding. Slump. A landslide characterized by a shearing and rotary movement of a generally independ- ent mass of rock or earth along a curved slip face (concave upward) and about an axis parallel to the slope from which it descends, and by backward tilting of the mass with respect to that slope so that the slump surface often exhibits a reversed slope facing uphill (Gary and others, 1972, p. 667). Stage. The height of a water surface above an established datum plane. Stage-discharge relation. The relation between gage height and the amount of water flowing in a stream channel. Stream competence. The measure of the ability of a stream to transport sediment. Strike valley. A valley formed by differential erosion of alternate layers of relatively resistant and nonresistant rocks, so that the valley coincides with the trend (strike) of the nonresistant layers and the bounding ridges coincide with the resistant layers. Talus. An accumulation of more or less angular rock fragments derived from and lying below a steep slope or cliff. Time of day. Expressed in 24-hour time. For example, 6:00 pm. is expressed as 1800 hours Mountain Daylight Time (MDT). Totals index. Defined as 2(Teso—T500)‘Deso where T850 is the 850 mb temperature, T500 is the 500 mb temperature and D850 is the 850 mb dewpoint depression, all expressed in °C. Unit discharge. The average number of cubic feet per second flowing from each square mile of area drained by a stream, assuming that the runoff is distributed uniformly in time and area. GLOSSARY vii PHOTOGRAPH CREDITS The authors wish to acknowledge the contribution of some of the figures used in this report. We are gratified that the following have given permission to use their photographs: John Asztalos, Waukesha, Wisconsin: figure 40. Eleanor J. Crosby, U.S. Geological Survey, Denver, Colorado: figures 90, 91, 92, 93, 94, 95, 96, 97, 98. Wallace R. Hansen, U.S. Geological Survey, Denver, Colorado: figures 65, 70, 73, 79, 83, 87, 89. Hogan and Olhausen, C. P., Loveland, Colorado: figures 58, 76, 99, 101, 102, 105, 109. William P. Rogers, Colorado Geological Survey, Denver, Colorado: figure 104. Paul W. Schmidt, U.S. Geological Survey, Denver, Colorado: figures 66, 67, 78B, 80, 85, 86. Robert Shelling, Loveland, Colorado: figures 78A, 82A- Ralph R. Shroba, U.S. Geological Survey Denver, Colorado: figure 68, 69, 71, 72, 75, 81, 82B, 88. James M. Soule, Colorado Geological Survey, Denver, Colorado: figures 103, 106, 107, 108. The Denver Post, Denver, Colorado: figures 63, 74, 77, 84. Timothy L. Wiley, Fort Collins, Colorado: figures 3A,3B. CONVERSION FACTORS The following factors may be used to convert U.S. customary units to Standard International (S. I.) units: Multiply To obtain U.S. customary units By metric units inch (in.) 2.54 centimeter (cm) 25.4 millimeter (mm) foot (ft) .3048 meter (In) .0003048 kilometer (km) yard (yd) .9144 meter (m) mile (mi) 1.609 kilometer (km) knot (kt) .5148 meter per second (m/s) Square mile (mi2) 2.590 square kilometer (km2) foot per second (ft/s) .3048 meter per second (m/s) foot per mile (ft/mi) .189 meter per kilometer (m/km) mile per hour (mi/h) 1.609 kilometer per hour (km/h) cubic foot per second .02832 cubic meter per second (m3/s) (ft3/s) cubic foot per second .01093 cubic meter per second per per square mile square kilometer ((ft3/s)/mi2) ((m3/s)/km2) cubic yard (yd3) .7646 cubic meter (m3) millibar (mb) .1 kilopascal (kPa) ton .9072 metric ton (t) Tgmperatures are converted from degrees Fahrenheit (F) to degrees Celsius (C) by the formula F=?C+32; from degrees Celsius (C) to Fahrenheit (F) by the formula C=% (F—32). Part A. Meteorology and Hydrology in the Big Thompson River and Cache la Poudre River Basins By jERALD F. MCCAIN oft/w U.S. GEOLOGICAL SURVEY and LEE R. HOXIT, ROBERT A. MADDOX, CHARLES F. CHAPPELL, and FERNANDO CARACENA oft/16 NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION STORM AND FLOOD OF JULY 31—AUGUST 1, 1976, IN THE BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS, LARIMER AND WELD COUNTIES, COLORADO GEOLOGICAL SURVEY PROFESSIONAL PAPER 1115 CONTENTS Abstract ................................................ Introduction ............................................ Acknowledgments ................................... The setting ............................................. Location ............................................ Terrain features related to flooding .................... Seasonal distribution of floods Precipitation ........................................ Meteorology of the storm ................................ Conditions prior to storm development ................ Conditions during the storm .......................... Analysis of conditions in region of flash flooding . . . . Physical models of thunderstorms ................. Rainfall analysis ................................. Hydrologic analysis of the flood ........................... Flood data .......................................... Station descriptions and streamflow data .......... Peak stages and discharges ....................... Velocities and depths ............................. Flood marks and profiles ......................... Detailed description of flood areas ..................... Big Thompson River basin: upstream from Olympus Dam ........................................ *U a: {N o wmmwwwwwu Hydrologic analysis of the flood—Continued Detailed description of flood areas—Continued Big Thompson River basin: Olympus Dam to Love- land Heights ................................. Big Thompson River basin: Loveland Heights to Drake ....................................... North Fork Big Thompson River basin: Glen Haven vicinity to Drake ............................. Big Thompson River basin: Drake to mouth of can- yon ......................................... Big Thompson River basin: mouth of canyon to South Platte River ................. 1 ......... Cache la Poudre River basin ....................... Flood frequency ...................................... The aftermath ........................................... The human element .................................. The damage ......................................... Comparison with previous rainstorms and floods ........... Sources of data .......................................... Gaging-station and miscellaneous-site data ................ Selected references ...................................... Index ................................................... ILLUSTRATIONS FIGURE . Map showing location of flood area in relation to west-central United States ...................................... . Map showing detailed location of flood area .................................................................... . Photographs showing typical view of terrain in the Big Thompson River basin near the storm center ................ . Graph showing relation of area to altitude for approximate storm area upstream from Drake ....................... . Map showing mean values of precipitation occurring during July in Colorado ..................................... . Map showing location of geographic features in Colorado and surrounding States referred to in the meteorological discussion ............................................................................................... 1 2 3 4. Map showing altitude range in area where flood originated ...................................................... 5 6 7 . Maps of the western conterminous United States, southwestern Canada, and northern Mexico showing: 8. Surface analysis, 0600 MDT, July 31, 1976 ............................................................ 9. 700-millibar analysis, 0600 MDT, July 31, 1976 ........................................................ 10. 500-millibar analysis, 0600 MDT, July 31, 1976 ........................................................ 11. Stability analysis, 0600 MDT, July 31, 1976 ........................................................... 12. Plot of rawinsonde data obtained at Denver, Colo., 0600 MDT, July 31, 1976 ...................................... 13. Plot of rawinsonde data obtained at Sterling, Colo., 0740 MDT, July 31, 1976 ..................................... 14. Map of Colorado and surrounding States showing regional surface analysis, 1200 MDT, July 31, 1976 ............... 15. Map of Colorado and surrounding States showing radar summary for 1135 MDT, with locations of fronts and squall line for 1200 MDT, July 31, 1976 .......................................................................... 16. Geostationary Operational Environmental Satellite photograph, 1200 MDT, July 31, 1976 ......................... 17. Plot of rawinsonde data obtained at Sterling, Colo., 1320 MDT, July 31, 1976 ..................................... 18. Map of Colorado and surrounding States showing regional surface analysis, 1400 MDT, July 31, 197 6 ............... 19. Map of Colorado and surrounding States showing radar summary for 1335 MDT, with locations of fronts and squall line for 1400 MDT, July 31, 1976 .......................................................................... III Page 62 63 64 65 66 67 69 70 70 70 71 72 72 81 83 Page 2 4 7 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 IV FIGURE 20. 21. 22. 23. 24. 25. 26. 27. 28—31. 32. 33—35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53—56. 57—59. 60. 61. CONTENTS Geostationary Operational Environmental Satellite photograph, 1400 MDT, July 31, 1976 ......................... Map of Colorado and surrounding States showing regional surface analysis, 1600 MDT, July 31, 1976 ............... Map of Colorado and surrounding States showing radar summary for 1535 MDT, with locations of fronts and squall line for 1600 MDT, July 31, 1976 .......................................................................... Geostationary Operational Environmental Satellite photograph, 1600 MDT, July 31, 197 6 ......................... Plot of rawinsonde data obtained at Sterling, Colo., 1602 MDT, July 31, 197 6 ..................................... Map of Colorado and surrounding States showing regional surface analysis, 1800 MDT, July 31, 1976 ............... Map of Colorado and surrounding States showing radar summary for 1735 MDT, with locations of fronts and squall line for 1800 MDT, July 31, 1976 .......................................................................... Geostationary Operational Environmental Satellite photograph, 1800 MDT, July 31, 1976 ......................... Maps of western conterminous United States. southwestern Canada, and northern Mexico showing: 28. Surface analysis, 1800 MDT, July 31, 1976 ............................................................ 29. 700-millibar analysis, 1800 MDT, July 31, 1976 ........................................................ 30. 500-millibar analysis, 1800 MDT, July 31, 1976 ........................................................ 31. Stability analysis, 1800 MDT, July 31, 1976 ........................................................... Plot of rawinsonde data obtained at Denver, Colo., 1800 MDT, July 31, 1976 ...................................... Graphs showing: 33. Time series of surface winds, altimeter setting, and surface-air and dewpoint temperatures, Stapleton Inter- national Airport, Denver, Colo., 1600—2300 MDT, July 31, 1976 ........................................ 34. Time series of winds, altimeter setting, and surface-air and dewpoint temperatures, Rocky Flats plant near Boulder, Colo., 1600—2300 MDT, July 31, 1976 ........................................................ 35. Time series of winds from the surface to a height of 2,000 feet, and surface-air and dewpoint temperatures, Table Mountain north of Boulder, Colo., 1630-2030 MDT, July 31, 1976 ................................. Maps showing radar and surface analyses at about 30-minute intervals for the Denver—Fort Collins, Colo., area, 1730—1900 MDT, July 31, 1976 ............................................................................ Interpolated plot of rawinsonde data for Loveland, Colo., 1800 MDT, July 31, 1976 ................................ Shaded-relief map of the Front Range in northeastern Colorado showing comparison of radar echoes observed at Limon and Grover, Colo ................................................................................... Diagram of the physical model of the thunderstorms over the Big Thompson River at 1845 MDT, July 31, 1976 ...... Photographs of a large thunderstorm several miles southwest of Lyons, Colo., at about 1830 MDT, July 31, 1976 . . . . Shaded-relief map showing total rainfall for July 31—August 2, 1976, along the Front Range in northeastern Colorado Graph showing cumulative rainfall at three stations in Boulder and Larimer Counties, Colo., July 31—August 2, 197 6 . Maps showing radar echoes in the Denver—Fort Collins, Colo., area, observed at Limon, Colo., from 1701 to 2200 MDT, July 31, 1976 ...................................................................................... Graph showing estimated cumulative rainfall at Glen Comfort and Glen Haven, Colo., July 31—August 1, 197 6 ....... Map showing stream-gaging stations and miscellaneous measurement sites in flood area in Larimer and Weld Coun- ties ..................................................................................................... Discharge hydrograph for the Big Thompson River at Estes Park (Site 1) ......................................... Discharge hydrograph for Fish Creek near Estes Park (Site 2) .................................................... Stage graph for the Big Thompson River near Estes Park (Site 3) ................................................ Photograph showing erosion along base of Olympus Dam caused by Dry Gulch floodwater ......................... Discharge hydrograph for the North Fork Big Thompson River at Drake (Site 21) until 2300 MDT on July 31, 197 6 . . . Discharge hydrograph for rising stage at the Big Thompson River at mouth of canyon, near Drake (Site 23) .......... Stage graph for Buckhorn Creek near Masonville (downstream site) .............................................. Discharge hydrographs for: 53. The Big Thompson River at mouth, near LaSalle (Site 28) .................................................. 54. The Cache la Poudre River at mouth of canyon, near Fort Collins (Site 34) ................................... 55. The Cache la Poudre River at Fort Collins (Site 36) ........................................................ 56. The Cache la Poudre River at mouth, near Greeley (Site 37) ................................................ Photographs showing: 57. Erosion damage to US. Highway 34 and to irrigation siphon at the gaging station on the Big Thompson River at month of canyon, near Drake (Site 23) .............................................................. 58. Aerial views of Drake ................................................................................... 59. Views of destroyed automobiles ......................................................................... Diagram showing comparison between July 31-August 1, 1976, rainfalls and previously observed rainfalls in eastern Colorado ................................................................................................ Diagram showing relation of peak discharge to drainage area for flood of July 31—August 1, 1976, and previous max- imum floods ............................................................................................. Page 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 44 45 46 47 49 50 57 58 62 63 63 64 65 66 67 68 69 69 70 72 73 74 75 76 CONTENTS V TABLES Page TABLE 1. Daily precipitation, in inches, Boulder and Larimer Counties, Colo ................................................... 46 2. Total rainfall, in inches, for July 31—August 2, 1976, at miscellaneous sites ........................................... 48 3. Flood stages and discharges in Larimer and Weld Counties for the flood of July 31—August 1, 1976, and during previous maximum floods ............................................................................................ 60 4. Hydrologic data for selected flood-data sites ....................................................................... 61 5. Damage estimates for the July 31-August 1, 197 6. flood ............................................................ 71 STORM AND FLOOD OF JULY 31-AUGUST 1, 1976, IN THE BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS, LARIMER AND WELD COUNTIES, COLORADO METEOROLOGY AND HYDROLOGY IN THE BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS By jERALD F. MCCAIN oft/1e U.S. GEOLOGICAL SURVEY, and LEE R. HOXIT, ROBERT A. MADDOX, CHARLES F. CHAPPELL, and FERNANDO CARACENA of the NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION ABSTRACT Devastating flash floods swept through the canyon section of Larimer County in north-central Colorado during the night of July 31—August 1, 1976, causing 139 deaths, 5 missing persons, and more than $35 million in total damages. The brunt of the storms oc- curred over the Big Thompson River basin between Drake and Estes Park with rainfall amounts as much as 12 inches being reported during the storm period. In the Cache la Poudre River basin to the north, a rainfall amount of 10 inches was reported for one locality while 6 inches fell over a widespread area near the cen- tral part of the basin. The storms developed when strong low-level easterly winds to the rear of a polar front pushed a moist, conditionally unstable airmass upslope into the Front Range of the Rocky Mountains. Orographic uplift released the convective instability, and light south- southeasterly winds at middle and upper levels allowed the storm complex to remain nearly stationary over the foothills for several hours. Minimal entrainment of relatively moist air at middle and up- per levels, very low cloud bases, and a slightly tilted updraft struc- ture contributed to a high precipitation efficiency. Intense rainfall began soon after 1900 MDT (Mountain Daylight Time) in the Big Thompson River and the North Fork Cache la Poudre River basins. A cumulative rainfall curve developed for Glen Comfort from radar data indicates that 7.5 inches of rain fell during the period 1930-2040 MDT on July 31. In the central part of the storm area west of Fort Collins, the heaviest rainfall began about 2200 MDT on July 31 and continued until 0100 MDT on August 1. Peak discharges were extremely large on many streams in the storm area—exceeding previously recorded maximum discharges at several locations. The peak discharge of the Big Thompson River at the gaging station at the canyon mouth, near Drake was 31,200 cubic feet per second or more than four times the previous max- imum discharge of 7,600 cubic feet per second at the site during 88 years of flood history. At the gaging station on the North Fork Big Thompson River at Drake, the peak discharge on July 31 was 8,710 cubic feet per second as compared to the previous maximum discharge during 29 years of record of 1,290 cubic feet per second. Peak discharges for three small tributaries near the area of heaviest rainfall northeast of Estes Park exceeded previously recorded max- imum discharges for basins of less than 4 square miles in Colorado. Stream velocities were rapid along the tributaries near the storm center and on the Big Thompson River in the canyon section, with average velocities of 20—25 feet per second being common. The flood crest on the Big Thompson River moved through the 7.7-mile reach between Drake and the canyon mouth in about 30 minutes for an average travel rate of 15 miles per hour, or about 23 feet per second. The peak discharge of the flood on the Big Thompson River at the canyon mouth exceeded the lOO-year flood discharge for the site by a ratio of 1.8. Upstream in the Big Thompson River basin, the flood was even more rare being 3.8 times the estimated 100-year flood discharge at the site on the Big Thompson River just upstream from Drake. In the Cache la Poudre River basin, recurrence inter- vals were computed to be 100 years for the flood on Deadman Creek and 16 years for Rist Canyon and the Cache la Poudre River at the canyon mouth near Fort Collins. Although the rainfall and flood discharges were unusually large, they are not unprecedented for some areas along the eastern foothills and plains of Colorado. The May 1935 and June 1965 floods on some streams along the eastern plains greatly exceeded the 1976 flood peaks in the storm area. Prior floods on several other streams in the foothills have approximately equaled the 1976 peak discharges. INTRODUCTION During the night of July 31-August 1, 1976, a com- plex system of thunderstorms produced intense rain- falls along the eastern foothills of the Rocky Moun- tains in northern Colorado (fig. 1). Devastating flash floods quickly swept through several streams in the area causing an appalling amount of death and destruction. The purpose of this report is to present an analysis of the genesis, growth, and culmination of the severe storm system and the disastrous floods which followed. Coming on the eve of Colorado’s 100th anniversary of statehood, the storm and flood quickly occupied the centerstage of attention. Centennial Sunday was still observed throughout most of the State but in a much subdued manner as the tragic news slowly filtered from a large area almost stripped bare of normal chan- nels of communication. During the ensuing days of 1 FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO 0.. <. \.. WYOMING "J ' '\. , COLORADO, '- ' ' Virginia Dal}. \_ / NORTH DAKOTA MONTANA SOUTH DAKOTA WYOMING NEBRASKA / eDenver COLORADO KANSAS OKLAHOMA NEW MEXICO TEXAS FIGURE 1.—Location of flood area in relation to west-central United States. METEOROLOGY, HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS 3 search, rescue, and readjustment, as the death toll and damage estimates continued to rise, it became obvious that the flood would be classified as Colorado’s worst natural disaster. The official tabulation by Larimer County officials lists 139 deaths, 5 persons reported as missing, and about $35.5 million in property damage. ACKNOWLEDGMENTS The data and interpretations contained in this report were obtained through the combined efforts of many individuals and local, State, and Federal agencies. This assistance, including factual information and financial support, is gratefully acknowledged. Because of the sparsity and destruction of recording instruments, much of the hydrologic and meteorologic data were provided by residents of the area. Also, ac- cess to private property was readily granted to the many field personnel working in the flooded area. Sincere appreciation also is extended to Larimer Coun- ty officials and to local municipal officials for their aid and patience during the data-collection period. The Colorado Department of Natural Resources, Division of Water Resources, Office of the State Engineer furnished funds to support data-collection ac- tivities and furnished streamflow data for gaging sta- tions operated by that agency. The Colorado Water Conservation Board of the same department developed flood and streambed profiles for the affected streams, assisted in the collection of other hydrologic data, and participated in the preparation of a basic flood-data report. Appreciation also is extended to the Colorado National Guard and private firms for providing helicopter transportation during the fieldwork. The Department of Atmospheric Science, Colorado State University, provided meteorological data for Fort Collins. Dr. C. Glenn Cobb supplied meteorological data for Greeley, measured at Ross Hall, University of Northern Colorado. John M. West of Rockwell International Corp. obtained the meteorological data for the Rocky Flats plant near Boulder. The National Center for Atmospheric Research Field Observing Facility obtained and sup- plied the invaluable sounding, radar, and surface data from the National Hail Research Experiment site in northeastern Colorado. The US. Army Corps of Engineers collected hydrologic data for parts of the area and provided estimates of flood damage. The Corps also provided funds for collection and reporting of hydrologic data. The US. Bureau of Reclamation assisted the Na- tional Weather Service in the collection of rainfall data and provided funds for the collection and analysis of hydrologic data. The Bureau also provided hydrologic data, including an independent estimate of the peak discharge at the mouth of Big Thompson Canyon. THE SETTING LOCATION The area from which the flood of July 31—August 1 originated lies in central Larimer County along the Front Range of the Rocky Mountains (fig. 2). The southern limit of flooding was near Estes Park about 50 miles northwest of Denver, while the northern limit was approximately 50 miles farther north near the Wyoming border. In the southern part of the storm area, the band of intense rainfall was only about 6 miles wide with the western edge located near Estes Park and the eastern edge located about 2 miles west of Drake. The western edge of the storm near the Wyoming border was located just west of Virginia Dale, and the eastern edge was about 10 miles east of Virginia Dale. According to information from the US. Army, Corps of Engineers (1976), rainfall of 5 inches or greater covered an area of more than 1,000 square miles during the storm period. The southern part of the flooded area is drained by the Big Thompson River and the northern part, by the Cache la Poudre River. Both streams flow in a general- ly eastward direction through Larimer and western Weld Counties to join the South Platte River in the vicinity of Greeley. Principal tributaries affected by flooding in the Big Thompson River basin were the North Fork Big Thompson River which enters at Drake, Buckhorn Creek which drains the northern part of the basin, and the Little Thompson River, which heads just southeast of Estes Park. The Little Thomp- son River flows eastward and generally parallels the main stem to the confluence near LaSalle. The North Fork Cache la Poudre River is the principal tributary of the Cache la Poudre River in the flooded area. Several small tributaries in the Bellvue area northwest of Fort Collins, including Rist and Soldier Canyons, also received severe flooding. TERRAIN FEATURES RELATED TO FLOODING The intense rains of July 31 fell on a part of the Col- orado Front Range, commonly referred to as the foothills area. This area is underlain by a complex assortment of metamorphic rocks of Precambrian age with numerous intrusives of igneous origin. Many faults and shear zones also complicate the bedrock geology of the area and appear to exert considerable 4 FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO 1 05° Base modified from US. Geological Survey 1:500,000, State base map, 1969 0 19 20 MILES l FIGURE 2.—Detai1ed location of flood area. control on the surface-drainage network because many The topography of the area is characterized by nar- stream courses generally coincide with faults. Almost row valleys bordered by side slopes generally ranging all stream valleys are covered by alluvium and slope- from 10 to 80 percent. Rugged rock faces of even wash material and the larger streams are bordered by steeper slope occur at many locations along the canyon gravel terraces and colluvium. floors and along the jagged ridges which rise as much METEOROLOGY, HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS 5 as 3,000 feet above the valleys (fig. 3). Soils, where pre- sent, are shallow, consisting of coarse material derived from both alluvial processes and from slope wash or colluvial processes. Soils generally grade from very gravelly and stoney near the ridges to a sandy to gravelly assortment near stream levels. Permeability of soils is rapid, ranging from 6 to 20 inches per hour with available water capacity generally less than 0.10 inch per inch. Soils are excessively drained with rapid runoff potential and are highly susceptible to severe erosion. N orth-facing slopes have a much denser forest cover than south-facing slopes, with Ponderosa pine most abundant at lower altitudes and Douglas-fir predominant near the mountain tops. Grasses and shrubs fill the open spaces between trees, being more abundant on south-facing slopes than on the denser forested north-facing slopes. Under the trees, the vegetation is sparse and much of the ground surface is exposed. Abundant growths of cottonwood, willow, and birch occur along the stream valleys where they are highly susceptible to the erosive process of the streams. The Big Thompson and the Cache 1a Poudre Rivers head near the same point on the Continental Divide at an altitude of about 11,000 feet. The altitude of the area from which the flood derived ranges from about 9,000 feet to about 7,500 feet as shown in figure 4. On both the Big Thompson and the North Fork Big Thompson Rivers, the western limit of flooding occur- red at an altitude of about 7,500 feet just west of Estes Park and Glen Haven, respectively. Downstream, the altitudes along the Big Thompson River range from 6,140 feet at Drake to 5,300 feet at the canyon mouth and 4,670 feet at the confluence with the South Platte River. Tributaries in the Big Thompson River basin near the storm center west of Drake range in altitude from about 7,000 feet to about 9,000 feet along the ridges. An area-altitude relation for the approximate storm area of 53 square miles in the Big Thompson River basin upstream from Drake is shown in figure 5. About 64 percent, or 33.5 square miles, lies in the range of 7,500—8,500 feet while the area above 8,500 feet comprises only 8.5 square miles, or about 16 per- cent of the total storm area in the Big Thompson River basin. Streambed gradients along the Big Thompson River average about 107 feet per mile in the canyon reach and about 10 feet per mile near the mouth at LaSalle. On the North Fork Big Thompson River, the average streambed gradient is 128 feet per mile in the reach between Glen Haven and Drake. Most of the small tributaries west of Drake are extremely steep with streambed gradients as much as 700 feet per mile. In the Cache la Poudre River basin, altitudes along the main stem are about 5,700 feet near the western limit of flooding at Poudre Park, 5,240 feet at the canyon mouth, and 4,610 feet at the mouth near Greeley. The streambed gradient from Poudre Park to the canyon mouth is about 46 feet per mile and about 9 feet per mile near the mouth. Altitudes along the North Fork Cache la Poudre River range from about 8,000 feet near the Wyoming border to 5,360 feet at the mouth. Streambed gradients on the North Fork are about 48 feet per mile in the northern part of the flood area and 43 feet per mile near the mouth. In the vicini- ty of Bellvue, small tributaries of the Cache la Poudre River head at about 8,000 feet. These small streams are fairly steep, averaging about 330 feet per mile. SEASONAL DISTRIBUTION OF FLOODS Three types of floods occur in the Colorado Front Range: snowmelt floods, floods produced by a com- bination of rain on snow, and rainfall floods. Snowmelt floods predominantly occur during May and June of each year and usually cause little or no damage. In fact, this type of runoff is usually welcomed as it is stored in off-channel reservoirs and provides a water supply during the dry summer months. Occasionally, low-intensity rainfall associated with frontal activity occurs over large areas of the Front Range hastening the snowmelt and producing severe flooding, especially on large streams. The third type, into which classifica- tion the July 31, 1976 flood falls, is the flash flood pro- duced by convective thunderstorms usually during the months of June, July, and August. Rainfall associated with this type of flooding is very intense and occurs in short periods. Surface runoff rapidly concentrates in nearby channels and flash flooding occurs in downstream areas. Both overland and stream velocities are swift, causing severe erosion along hillsides and in streams. Property damage is usually high and fatalities frequently occur. The short period of time between the intense rainfall and flash flooding frequently precludes advance warning to downstream areas. Often associated with this type of flood is the reported “virtual wall of water.” In almost all aspects, the flash flood is the most dangerous of the three types of floods. PRECIPITATION A smoothed analysis of the average precipitation oc- curring during July in Colorado is shown in figure 6 (National Oceanic and Atmospheric Administration, 1973). Typically, summer precipitation in north- eastern Colorado is light and comes from afternoon and early evening thunderstorms that form over the mountains and move eastward over the plains. While OCIV'HO'IOO 'HEIAIH NOSdWOHL {)18 91.61 ‘I LSDDflV-IS X'Iflf ‘GOO’IJ gmemOwObOQ<. mw‘UwObOO—Mfi wfio‘. HEOZ—meZ wH26 o>omm b> vOGme Haw/NEW w>wHZm FIGURE 3.—Typical views of terrain in the Big Thompson River basin near the storm center. mHOOU. udb< mHI>CQCm€ H. 53. mum emogwmoz Nixmw. OOFON>UO FIGURE 3.—Continued. METEOROLOGY, HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS 9 .16 Red Edges Base modified from US. Geological Survey 1500.000, State base map, 1969 (I) 1? Wiper: Hurrxch Greeley No aceweHO % Ear Grlcrest 20 MILES l CONTOUR INTERVAL 500 FEET FIGURE 4,—Altitude range in area where flood originated. large amounts of rain and hail often fall with these storms, the precipitation is localized and of short dura- tion. The 6—12 inches of rainfall observed on the eve- ning of July 31, 1976,was several times the average monthly value for July in northeastern Colorado. METEOROLOGY OF THE STORM Although the intense rainfall was confined to a nar- row band along the foothills in northeastern Colorado, meteorological processes on a much larger scale were 10 FLOOD, JULY 31-AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO 9000 V I I I I I 8500 8000 ~ 7500 — ALTITUDE, IN FEET ABOVE NATIONAL GEODETIC VERTICAL DATUM OF 1929 7000 I 6500 I I I I I A 0 10 20 30 40 50 60 AREA ABOVE GIVEN ALTITUDE, IN SQUARE MILES FIGURE 5.—Relation of area to altitude for approximate storm area upstream from Drake. responsible for creating the thunderstorms that caused the floods. Surface and upper-air data, stability analyses, rawinsonde data, radar summaries, and satellite photographs are used in summarizing the meteorological events. Geographic locations mention- ed in the discussions are shown in figure 7. CONDITIONS PRIOR TO STORM DEVELOPMENT Atmospheric conditions for western North America at 0600 MDT (Mountain Daylight Time) on July 31, 1976, are shown in figures 8—11. A strong polar high- pressure area was centered in southern Canada. A dou- ble frontal structure extended from the Great Lakes through Kansas and then northwestward into central Montana and defined the southern boundary of the polar air. The leading front was characterized by a wind shift and pressure trough while the trailing front was characterized by a pressure trough with a strong thermal gradient. To the west of the fronts, a weak low-pressure area was located over western Colorado (fig. 8). Surface-dewpoint temperatures equal to or greater than 60°F extended northwestward from Kansas into Colorado and Nebraska. A narrow band of very moist surface air with dewpoints equal to or greater than 65°F was moving into southwestern Nebraska behind the trailing front (fig. 8). In general, surface dewpoints were 5°—15°F higher than normal over much of the in- termountain west and central High Plains. At the 700—millibar (fig. 9) and 500-millibar levels (fig. 10), the dominant large-scale feature was a ridge extending from southern Texas into southwestern Canada. Moisture values were high over much of the area west of the pressure ridge. A weak trough extend- ed from Utah to southern New Mexico (fig. 9). At the 500-millibar level, the weak trough was farther south, located over Arizona and New Mexico. A second weak trough at the 500-millibar level was located over northern Mexico (fig. 10). The stability analysis (fig. 11) shows the Totals In- dex and Lifted Index for 0600 MDT, July 31, 1976. Values of the Totals Index equal to or greater than 46 indicate favorable conditions for convective develop- ment and values greater than 50 indicate potential for moderate to heavy thunderstorms (Miller, 1972). The Lifted Index was computed for a parcel of air with mean-thermodynamic characteristics of the lowest 100-millibar layer. Negative values indicate a condi- tionally unstable environment. Both indices showed the potential for moderate to heavy thunderstorms over northern Arizona, most of Utah and Nevada, western Kansas, and northeastern Colorado. Rawinsonde data obtained during the early morning of July 31, 1976, at Denver and Sterling, Colo., are shown in figures 12 and 13. The data for Denver at 0600 MDT (fig. 2) indicate that the air was very moist with an average mixing ratio for the lowest 100-millibar layer of 12 g/kg (grams per kilogram) below a temperature inversion at the 670-millibar level. Winds above the inversion were light and variable while winds in the cool airmass below the in- version were generally easterly with speeds less than 10 knots. The Lifted Index was —1, but the level of free convection was at the 530-millibar level indicating considerable lifting and (or) heating would be needed to initiate deep convection. The high moisture content of the air was the most unusual feature of the rawinsonde data. Precipitable water contents of 0.67 inch in the lowest 150-millibar layer, and 1.00 inch in the layer from the surface to the 500-millibar level were approx- imately 50 percent greater than the means for July at Denver of 0.40 inch and 0.69 inch, respectively (Lott, 1976). A low overcast at 1,200 feet was reported at Denver at the time of the rawinsonde observation. The rawinsonde data obtained at Sterling, Colo., at 0740 MDT (fig. 13) was part of the National Hail Research Experiment. A pronounced radiational inver- sion near the land surface was topped by a weaker in- version at the 725-millibar level. Winds in the cool air- mass were easterly with speeds less than 10 knots. The METEOROLOGY, HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS 1 1 109° 41° '7‘ I??? 107° 106° 105° 104° 103° -— , I “r | I Area of regional analysis Crai ‘ . 9 Sterling o «r— I 2.0 2-5 3.0 Grand Junction 1.0 1.5 2.0 o 391— __ Springs 0 o Pueblo 38° :_ La Junta J 0 Durango .._l . I L U I W I 50 l . Trinidad 100 MILES I EXPLANATION —2.0— LINE OF EQUAL MEAN PRECIPFTATION FOR JULY—Interval 0.5 inch FIGURE 6.—Mean values of precipitation, in inches, occurring during July in Colorado (from National Oceanic and Atmospheric Administration, 1973). Lifted Index was +1 and the level of free convection was at the 480-millibar level. The average mixing ratio in the lowest 100 millibars was 11 g/kg. Precipitable water contents of 0.59 inch in the lowest 150-millibar layer and 1.04 inches in the layer from the surface to the 500-millibar level were similar to the amounts oc- curring at Denver, Colo. In summary, the morning analyses indicated that the stage was set for significant thunderstorm activity over a large area of the West. Abundant moisture, a conditionally unstable thermal structure, and weak vertical motions driven by the northward-moving pressure trough were the major features of this en- vironment. The changing meteorological conditions during the afternoon of July 31, 1976, are shown in figures 14—24. A surface analysis, radar summary, and Geostationary Operational Environmental Satellite (GOES) photograph are presented for 2-hour intervals begin- ning at 1200 MDT and ending at 1800 MDT. After- noon rawinsonde data taken at Sterling, Colo., supple- ment the analyses. To make use of all available data, aircraft altimeter settings were used to define the sur- face pressure fields. 12 FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO 115° 110° 105° 100° 95° I ‘ l l l \ Hufon Rapid City Lander Casper 0 0 Rock Springs North 3 . _ Laramie. Cheyenne Sid.nev ”a.“ Salt Lake Billghfld‘fifiSG’HWJW”?o’r‘f‘Collins ‘é'G’rover' X 40., 0 ‘ i o ' \ , 40 \ City 5 anyon Estes Park\o/<‘3€f;'°Loveland OSterling Mywwww... f E l >A¢Boulder ’Akm” ._ M « i. fGrand fig.” , /\ 'Denvef ‘ Junction ,//Y\ALImOH/\, 0Goodland - w K “A ,Palmer Ridge { olorado Springs / , Pueblo ’/ AA Arkansas D o d qE Citv —\ /\Tr1nidad River - l A ' _/ ,, ‘Er’ont an e Continental 9 Divide 350‘ Winslow Amarillo ’ 350 O U Albuqiierque 1 1 1 110° 1050 1000 200 O 100 l l 300 400 MILES 1 1 1 FIGURE 7,—Location of geographic features in Colorado and surrounding States referred to in the meteorological discussion. By 1200 MDT, the trailing front had moved into ex- treme northeastern Colorado (figs. 14, 15). Behind this front the winds were easterly with gusts as much as 25 knots. Dewpoint temperatures remained equal to or greater than 60°F to the north of the leading front. During the afternoon, the leading front remained almost stationary along the foothills in northeastern Colorado while the trailing front moved southwestward at 15—20 knots. The cloud cover over the area at 1200 MDT is shown in figure 16. At 1320 MDT, a second rawinsonde was released at Sterling, Colo. (fig. 17). By this time, the trailing front was located 15—30 miles southwest of Sterling. The data obtained from the rawinsonde were considerably different from those data obtained earlier in that morn- ing by the rawinsondes released from Denver and Ster- ling, Colo. Low-level moisture content had increased; the average mixing ratio of the lowest 100-millibar layer was 13.8 g/kg, an increase of 2.8 g/kg. The Lifted Index had decreased from + 1 to —4, and the lifted con- densation level had decreased to the 780-millibar level. More importantly, the level of free convection had decreased from the 480-millibar-level to the 640-millibar level. Precipitable water contents of 0.78 inch in the lowest 150-millibar layer and 1.3 inches in the layer from the surface to the 500-millibar level were almost double the mean July values for Denver, Colo. Below the inversion at the 720-millibar level, the winds were easterly at 10—15 knots. Above the inversion, the winds were westerly but light, indicating that Sterling, 0010., was almost directly below the upper-level ridge. The air behind the trailing front was characterized by a METEOROLOGY, HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS 13 130° 125° 120° 115° 110° 105° 100° 95" 90° 85° 80° / / I I I | .\ I \ \ \ CA \ 1818 20 22 24 / \24 22 20 18 / 50° 50° 45° \ 40° \ 35° \ 30° \ 25° \ 0 500 MILES L—I—l—I—L—l 60 65 7o 75 EXPLANATION AREA WHERE DEWPOINT TEMPERATURE EXCEEDED 60 —'—'- COLD FRONT. —Dashed where dissipating DEGREES FAHRENHEIT —'—-‘— STATIONARY FRONT —Dashed where d' s' at‘ . IS I In LINE OF EOUAL DEWPOINT TEMPERATURE. —Imerva| 5 p 9 degrees Fahrenheit —"— SUUALL LINE LINE OF EOUAL AIR TEMPERATURE. —Interva| 5 degrees H GEOGRAPHIC CENTER OF HIGH-PRESSURE SYSTEM Fahrenheit LINE OF EOUAL ATMOSPHERIC PRESSURE AT SEA L GEOGRAPHIC CENTER OF LOW-PRESSURE SYSTEM LEVEL. —12=1012 millibars. Interval 2 rnillibars; l-millibar interval shown by dashed line FIGURE 8.—Surface analysis, 0600 MDT, July 31, 1976. deep, unusually moist boundary layer which was condi- During the afternoon, the surface pressures were tionally very unstable but which would have to be steady or increasing slightly over much of Nebraska lifted about 4,000 feet to release its instability. and were decreasing in western Colorado (figs. 18, 21), 14 FLOOD, JULY 31-AUGUST 1. 1976, BIG THOMPSON RIVER, COLORADO 130° 125° 120° 1 15° 110° 105° 100° 95° 90° 85° 80° l | l \ \ 50° 45° 40° 35° 30° 25° \ 313 318 25° 12 186 ':-'191 M 13 01 —02 “ I I \ \ 1 15° 110° 105° 100° 95° 90° 0 500 MILES I—L_—l—;_J—' EXPLANATION L GEOGRAPHIC CENTER OF LOWEST ALTITUDE OF AREA WHERE DEWPOlNT TEMPERATURE WAS WITHIN 6 700_MILLIBAR SURFACE DEGREES CELSIUS OF AIR TEMPERATURE —316—— CONTOUR SHOWING ALTITUDE 0F 7OD-MILLIBAR iii—431 OBSERVATION STATION—Upper left number is air SURFACE.—-316=3160 meters. Contour interval 20 temperature, in degrees Celsius. Lower left number is meters; iO-meter interval shown by dashed contours. depression of dewpoint temperature below air Datum is mean sea level temperature, in degrees Celsius; X shown when depression greater than 30 degrees Celsius. Upper right “10—— LINE 0_F EQUAL AIH TEMPERATURE—Interval 2 degrees number is altitude of 7OD-millibar surface, in meters; CE'S'US 143=3143 meters. Lower right number is 12-hour _ _ altitude change of 700-millibar surface, in meters; AXIS 0F PRESSURE THOUGH -Ul=~10 meters. Shaft indicates wind direction. Barbs on shaft indicate wind speed, in knots. Long H GEDGRAPHlC CENTER 0': HIGHEST ALT'TUDE 0F barb=10knots;shortbarb=5knots.LVinplaceofshaft 700'Mll-L'BAR SURFACE and barbs indicates wind speeds were less than 3 knots and direction was variable. M indicates missing data FIGURE 9.—700-millibar analysis, 0600 MDT, July 31, 1976. METEOROLOGY, HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASIN S 130° 125° 120° 1 15° 110° 105° 100° 95° 90° 85° 80° 0 500 MILES I—J—J—l—I__J EXPLANATION AREAWHERE DEWPOINTTEMPERATUREWAS WiTHING -07 537 OBSERVATION STATION. —Upper left number is air DEGREES CELSIUS OF AIR TEMPERATURE 3 —01 temperature, in degrees‘Celsius. Lower left number is CONTOUR SHOWING ALTITUDE 0F 500-MlLLIBAR depress” °T dewpmm temperature ”em” a” _ , temperature, In degrees CeISIus, X shown when SURFACE _ 5,32_5820 meters. Contour Interval 30 depression greaterthan 30 degrees Celsius. Upper right meters. Datum '5 mean sea level number is altitude of SOD-miilibar surface, in meters; -———13—— LINE OF EQUAL AIR TEMPERATURE. —interval2degrees 581:5”) "'9‘9'5- “W".E'ght "umbe' 1'5 mm" Celsius aItItude change of 500-mIIIIbar surface, In meters; —01=-10 meters. Shaft indicates wind direction. ——— AXIS 0F PRESSURE TROUGH Barbs on shaft indicate wind speed, in kndts..Long H GEOGRAPHIC CENTER OF HIGHEST ALTITUDE 0F bmtzgigilgodaigots, 5““ ham—5 "nms' M '"d'me‘ SOD-MILLIBAR SURFACE FIGURE 10.—500-millibar analysis, 0600 MDT, July 31, 1976. 15 16 FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO 125° 120° 1 15° 1 10° 105° 100° 95° 90° 85° 80° 7 l l I \ \ \ m 033 500 O46 O45 45° 40° 35° 30° 25° .' | l l \ 115° 110° 105° 100° 95° 90° 0 500 MILES EXPLANATION AREA WHERE TOTALS INDEX EXCEEDED 50 of? OBSERVATION STATION. —Upper number, where shown, is Totals Index. Lower number, where shown, is Lifted _ ._ , Index. X indicates Totals Index andlor) Lifted Index not 46 LINE OF EQUAL TOTALS INDEX—Interval 2 units determined. E indicates estimated value. M indicates -—-~2~—~— LINE OF EQUAL LIFI'ED INDEX. —lnterva| 2 units missing data —'—'—' COLD FRONT. —Dashed where dissipating —'—-‘— STATIONARV FRONT. —Dashed where dissipating —' - —- SOUALL LINE FIGURE 11.—Stabi1ity analysis, 0600 MDT, July 31, 1976. 100 150 200 250 300 PRESSURE, IN MILLIBARS 400 500 600 700 800 900 1000 METEOROLOGY, HYDROLOGY. BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS 17 "‘"Totals mdex = 47 E /< \\ \ \\\/\ \/ \/ \/ V Lifted index: —1 _W\ \V\ \/ W \/ h—~ EXPLANATION <—— DEWPOINT TEMPERATURE WIND—DIRECTION AND SPEED OBSERVATION—Shaft indi- AIR TEMPERATURE cates wind direction; north is —O— AIR-TEMPERATURE SCALE, IN DEGREES CELSIUS at top. Barbs on shaft indicate _ __ wind speed, in knots. Long 3033 DRY-ADIABAT SCALE, IN DEGREES KELVIN barb=10 knots; short barb=5 —-—10-——- MOIST-ADIABAT SCALE, IN DEGREES CELSIUS knots. FIGURE 12.—Plot of rawinsonde data obtained at Denver, Colo., 0600 MDT, July 31, 1976. 18 PRESSURE, IN MILLIBARS 100 150 200 250 300 ' 400 500 600 700 800 900 1000 FLOOD, JULY 31—AUGUST 1, 1976. BIG THOMPSON RIVER, COLORADO % “a 1% \V / 93> \10 14 18 22 \ 26\ 30 34 \ \ MINE/ff CH x5 Lifted index = +1 — Totals index = 44 / 95% />\ "7 \ if?” V VW V V \W\\/ /\\X V\\/ \m FIGURE 13.- EXPLANATION DEWPOINT TEMPERATURE WIND-DIRECTION AND SPEED OBSERVATION—Shaft indi- AIR TEMPERATURE cates wind direction; north is AIR-TEMPERATURE SCALE, IN DEGREES CELSIUS suntan-Barb: on staftindicate win spee , in nots. ong DRY-ADIABAT SCALE, IN DEGREES KELVIN barb=10 knots; short barb=5 MDIST-ADIABAT SCALE, IN DEGREES CELSIUS knots- Plot of rawinsonde data obtained at Sterling, Colo., 0740 MDT, July 31, 1976. METEOROLOGY, HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS 115° 110° 105° 100° 95° / __ l l l 5 \ 30,26 64 8 69 30.19—76 48 13/ 54 w z / ‘ ' 30.0], ,A [11% 9 , 34‘ 67 ”so. 76 57 01/3/ 20 60 30*:3 40° gig @3009 30.10%; 35° ~ . 81 mm”? 5 88, 30.08 90 30-03 53 : 53 I . | . 1 10° 105° 100° 0 100 200 300 400 MILES I_ I I I I EXPLANATIGN 60 65 70 75 AREA WHERE DEWPOINT TEMPERATURE EXCEEDED L GEOGRAPHIC CENTER OF LOW-PRESSURE SYSTEM 60 DEGREES FAHRENHEIT \ OBSERVATION STATION—Upper left number is air LINE OF EQUAL DEWPQ‘NT TEMPERATURE—Interval 73330.23 temperature, in degrees Fahrenheit. Lower left 5 degrees Fahrenheit 554 ‘4" number is dewpoint temperature, in degrees ——30 15— LINE OF EOUAL ALTIMETER SETTING IN INCHES OF Fahrenheit “We“ ”9'“ numb.” is a'timetef 39“”‘9' ' MERCURY —lnterval 005 inch ' In inches of mercury. Lower rIght number IS 3—hour ' ' pressure change, in tenths of a millibar; <=de- —'—'— COLD FRONT. —Dashed where dissipating crease, f=increase,and / =no change. Shaft indi- cates wind direction. Barbs on shaft indicate wind ' STATIONARY FRONT speed, in knots. Long barb=10 knots; short barb=5 _. ._ SOUALL LINE knots. ©=calm winds, less than 2 knots. Additional symbols and notes explained below: H GEOGRAPHIC CENTER OF HIGH—PRESSURE SYSTEM K =Thunder heard, but no precipitation at station I? =Slight rain shower(s) GZD=Peak wind gust observed during last hour FIGURE 14.—Regional surface analysis. 1200 MDT, July 31, 1976. 19 40° 35° 20 115° 110° 105° FLOOD, JULY 31-AUGUST 1, 1976, BIG THOMPSON RIVER. COLORADO 100° 95° / I I N0 ECHOES (DRW 40° 1 N0 ECHOES I \ 1N0 ECHOESHM , 40° a , Go 0 i o . , 35° 35° ~ i 0 O 0 OTRW— N0 ECHOES I ' I I 110° 1050 1000 o 100 200 300 400 MILES | l | | l EXPLANATION AREA OF RADAR ECHOES. —Usua||y interpreted as an O) SCATTERED ECHOES 'N AREA area with precipitatlon A470 LOCATION OF ECHO.—A=estimated; 470=maximum . . . / altitude of top of echo, in hundreds of feet. COLD FRONT —Dashed where dISSIpatIng 470:“,000 feet STATIONARY FRONT TRW MODERATE THUNDERSTORM SOUALL LINE TRW+ + VERY HEAVY THUNDERSTORM GEOGRAPHIC CENTER OF HIGH-PRESSURE SYSTEM TRW- THUNDERSTORM. APPARENTLY WEAKENING GEOGRAPHIC CENTER OF LOW-PRESSURE SYSTEM RW MODERATE RAIN SHOWER D'RECTION (ARROW) AND SPEED (NUMBER) OF RW-/+ LIGHT RAIN SHOWER, INCREASING IN INTENSITY ECHOES —Speed. In knots nwu RAIN SHOWER, INTENSITY UNKNOWN ISOLD ISOLATED FIGURE 15.—Radar summary for 1135 MDT, with locations of fronts and squall line for 1200 MDT, July 31, 1976. METEOROLOGY, HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS 21 FIGURE 16.—Geostationary Operational Environmental Satellite photograph, 1200 MDT, July 31, 1976. Bright areas are clouds. where surface temperatures peaked at 90°F or more. A combination of increasing temperatures and weak dynamics associated with the northward-moving pressure trough are believed to be responsible for the lower pressures west of the Continental Divide. The pattern of increasing and decreasing pressures con- tributed to an increase in the east-to-west pressure gra- dient across northeastern Colorado. The radar summaries and satellite photographs (figs. 15, 16, 19, 20, 22. 23) show that deep convection developed over a large part of the Rocky Mountains and central High Plains. Several large thunderstorms occurred along and just north of the trailing front in eastern Colorado and southern Kansas. A large area of showers and thunderstorms also developed over the mountains of northern New Mexico and southwestern Colorado. In southern Utah, a well-defined squall line was moving north-northeastward at 20—25 knots. In the late afternoon, widespread convective activity also spread over east-central Wyoming. Generally, the storms that developed in western Kansas and eastern Colorado and Wyoming were slow-moving or sta- tionary, while those to the west of the Continental Divide moved to the north or northeast at 15—25 knots. By 1600 MDT the trailing front had merged with the leading front over most of Kansas and was only 30—50 miles east of the foothills in northeastern Colorado (fig. 21). At this time, scattered cumulus and towering cumulus clouds were over the foothills area of northeastern Colorado, which includes the drainage areas of the Big Thompson and the Cache la Poudre Rivers, but little or no precipitation fell (figs. 22, 23). Scattered thunderstorms were forming along the northern slopes of the Palmer Ridge southeast of Den- ver, Colo., and moderate convective activity had devel- oped in the mountains of north-central Colorado. At 1602 MDT, a third rawinsonde was released at 22 PRESSUREIN MHUBARS 100 150 250 300 400 500 600 700 800 900 1000 FLOOD, JULY 31-AUGUST 1. 1976, BIG THOMPSON RIVER, COLORADO <0 \\ \ i\\ I \ v“ \ \ \ I \( /Liftedindex=—4 l \ \VMW\ ' éfi _Totalsindex=52 /‘< \\ ‘\\\\\/\\/i I \ I EXPLANATION <— DEWPOINT TEMPERATURE WIND—DIRECTION AND SPEED OBSERVATION. —Shaft indi- AIR TEMPERATURE cates wind direction; north' Is —---0— AIR-TEMPERATURE SCALE, IN DEGREES CELSIUS at tcép. Barb: on shaftindi-cate win spee , in knots. ong —303_3— DRY—ADIABAT SCALE, IN DEGREES KELVIN barb=10 knots; short barb=5 —-—10—~ MOIST-ADIABAT SCALE, IN DEGREES CELSIUS knOIS- FIGURE 17.——Plot of rawinsonde data obtained at Sterling, Colo., 1320 MDT, July 31, 1976. METEOROLOGY, HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASIN S 115° 110° 105° 100° 95° 40° 35° - i / ,/~ , ' '56,~<’36.21 53 \ I I \ 30.20 81 A 30.15 \"V, 30.19 1 1 Ma. 62 30' 01753040; 60 65 70 75 —30.15— 30.10 30.05 a 3 .00 98 29.93 89 30.07 gym“ ., \4? > i ‘ , ‘ “0° 105° 1000 O 100 | EXPLANATION AREA WHERE DEWPOINT TEMPERATURE EXCEEDED 60 DEGREES FAHRENHEIT LINE OF EOUAL DEWPOINT TEMPERATURE. —|nterval 5 degrees Fahrenheit LINE OF EOUAL ALTIMETER SETTING, IN INCHES OF MERCURY. —|nterval 0.05 inch COLD FRONT. —Dashed where dissipating STATIONARY FRONT SOUALL LINE GEOGRAPHIC CENTER OF HIGH-PRESSURE SYSTEM 300 I L 99,3031 57w .4 400 MILES I GEOGRAPHIC CENTER OF LOW-PRESSURE SYSTEM OBSERVATION STATION—Upper left number is air temperature, in degrees Fahrenheit. Lower left number is dewpoint temperature, in degrees Fahrenheit. Number at right is altimeter setting, in inches of mercury. Shaft indicates wind direction. Barbs on shaft indicate wind speed, in knots. Long barb=10 knots, short barb=5 knots.©>=calm winds, less than 2 knots. Additional symbols and notes explained below: it =Thunderstorm with rain FIGURE 18.—Regional surface analysis, 1400 MDT, July 31, 1976. 23 40° 35° 24 FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO 115° 110° 105° 100° 95° 7&5er _ C ' ‘ ‘ ‘ , " ‘ ‘ NO ECHOES 2%.: (DTRW y —*25 ‘—"25 4 . NE” , f; 0 40° _ NO ECHOES ” 1 ’40 9 35° 35° g... I 110° 105° 100° 0 100 200 300 400 MILES l | l | 4] EXPLANATION AREA OF RADAR ECHOES. —Usua||y interpreted as an TRW‘ ”GHT THUNDERSTORM area with precipitation TRW MODERATE THUNDERSTORM AREA WHERE RADAR ECHDES ORIENTED IN A LINE TRW+ HEAVY THUNDERSTORM COLD FRONT. —Dashed where dissipating TRW+ + VERY HEAVY THUNDERSTORM STATIONARY FRONT TRW/+ MODERATE THUNDERSTORM, INCREASING IN SOUALL LINE 'NTENS'TY TRW++/+ VERY HEAVY THUNDERSTORM, INCREASING IN GEOGRAPHIC CENTER OF HIGH-PRESSURE SYSTEM INTENSITY R— LIGHT RAIN GEOGRAPHIC CENTER OF LOW-PRESSURE SYSTEM R-/+ LIGHT RAIN INCREASING :N INTENSITY DIRECTION (ARROW) AND SPEED (NUMBER) OF _ ECHOES.——Speed, in knots RW LIGHT RAlN SHOWER (D SCATTERED ECHOES IN AREA RW MODERATE RAIN SHOWER ® BROKEN AREAS WITH ECHDEs RW—/+ LIGHT RAIN SHOWER, INCREASING IN INTENSITY 570 LOCATION OF ECHO.—570=maximum Observed al- I'M RADAR OPERATING IN UMITED MODE ' titude of top of echo, in hundreds Of feet. 570=57,000 ISOLD ISOLATED feet FIGURE 19.—Radar summary for 1335 MDT, with location of fronts and squall line for 1400 MDT, July 31, 1976. METEOROLOGY, HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASIN S 25 FIGURE 20.—Geostationary Operational Environmental Satellite photograph, 1400 MDT, July 31, 1976. Bright areas are clouds. Sterling, Colo. (fig. 24). By this time, the trailing front was located about 75 miles southwest of Sterling. The rawinsonde recorded a mean mixing ratio of 12.5 g/kg, a Lifted Index of -2, and a level of free convection at the 600-millibar level. Evidently, the air with the greatest potential for strong convection had moved southwestward in a narrow band behind the trailing front. Conditions that existed at 1800 MDT while the storms that caused the flash flooding were forming are depicted in figures 25—32. In western Colorado, the surface low had reached its maximum intensity. The trailing front had moved into the foothills in north- eastern Colorado and had merged with the leading front everywhere except in the Arkansas River valley in southeastern Colorado. Easterly surface winds behind the trailing front were 15—30 knots in a broad band from central Kansas to eastern Wyoming. At the 700-millibar and 500-millibar levels (figs. 29, 30), the large ridge had increased in amplitude over Montana and southwestern Canada. The trough at the 700-millibar level had moved only slightly while the two troughs at the 500-millibar level had evolved into a single northward-moving trough extending from cen- tral Nevada to northern New Mexico. The radar sum- mary for 1735 MDT (fig. 26) and the satellite photograph for 1800 MDT (fig. 27) indicated that the squall line in Utah and most of the widespread thunderstorms over the mountains of New Mexico and Colorado were alined along and to the northeast of the trough at the 500-millibar level. The stability analysis for 1800 MDT (fig. 31) in- dicated that the area having the thermodynamic potential for strong thunderstorms had increased dur- ing the day. Very unstable conditions extended from northern New Mexico to Montana. For operational purposes, rawinsonde are released about 45 minutes prior to the 0600 and 1800 MDT standard upper-air analyses time. On the evening of July 31, the rawinsonde from Denver, 0010., was released at 1715 MDT. Data from that rawinsonde are shown in figure 32. Diurnal heating had modified the airmass over Denver, Colo., significantly. The inver- sion had risen to the 590-millibar level with the lapse rate below the inversion near dry adiabatic. The mean mixing ratio in the lowest 100-millibar layer had decreased from 12.0 to 9.5 g/kg; the Lifted Index was —2. Precipitable water in the lowest 150-millibar layer 26 FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO 115° 1100 1050 1000 950 I l I 1 ‘ \ W \ g ' s? 30H W I‘d ” § 3020 30.15 gab 230108 30.1 Igngzz: I36? 30.10 . 40° 35° _‘ G'JJ %”"m 3020 ' 80 3014 54“ -. 40° 0030.00 ' r 300 20 , ‘i 54 +0002}; 92 Q30 03 3000‘ 35° —30.15— \030 M 95 30.02 93 30.00 55 . 90 30. 04 30.10 30. 055 4Q _: N I I 110° 105° 100° 0 100 200 300 400 MILES l l 1 l J EXPLANATION AREA WHERE DEWPOINT TEMPERATURE EXCEEDEO L GEOGRAPHIC CENTER OF LOW-PRESSURE SYSTEM 60 DEGREES FAHRENHEIT 82' 30 II OBSERVATION STATION—Upper left number is air LINE OF EQUAL DEWPOINT TEMPERATURE. —|nten/a| 55%} temperature, in degrees Fahrenheit. Lower left 5 degrees Fahrenheit number is dewpoint temperature, in degrees Fahrenheit. Number at right is altimeter setting, in ”'35? EgyALIALTIBAIEJEE .STING' IN INCHES 0F inches of mercury. Shaft indicates wind direction. CU '— nterva ' "10 Barbs on shaft indicate wind speed, in knots. Long COLD FRONT. —Dashed where dissipating barb=10 knots, short barb=5 knots.©=calm winds, less than 2 knots. Additional symbols and notes STATIONARY FRONT eprained below; SQUALL LINE ‘i =Thunder heard, but no precipitation at station “a =Slight rain showerls) x =Lighting observed GZZ=Peak wind gust observed during last hour GEOGRAPHIC CENTER OF HIGH-PRESSURE SYSTEM FIGURE 21.—Regional surface analysis. 1600 MDT, July 31, 1976. METEOROLOGY, HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS 27 105° 100° 95° N0 ECHOES TRW ISOLI 40° I / 40° 500 TRW TRW+/+ .I —— '7'- TRWx/+ / , ,_ 10 35° , 35° NO ECHOES I 100° 200 300 400 MILES I I I EXPLANATION AREA OF RADAR ECHOES. —Usually interpreted as an 430 LOCATION OF ECHO. —430=maximum observed al- area with precipitation titude of top of echo, in hundreds offset. 430=43,000 . . . 0/— feet COLD FRONT —Dashed where dIsSIpatIng TRW MODERATE THUNDERSTORM STATIONARY FRONT TRW+ HEAVY THUNDERSTORM —- -— SOUALL LINE TRW+ + VERY HEAVY THUNDERSTORM H GEOGRAPHIC CENTER OF HIGHPRESSURE SYSTEM TRW+/+ HEAVY THUNDERSTORM, INCREASING IN INTENSITY L GEOGRAPHIC CENTER OF LOW-PRESSURE SYSTEM TRWXI+ INTENSE THUNDERSTORM, INCREASING IN INTEN- SITY —>25 DIRECTION (ARROW) AND SPEED (NUMBER) OF _ ECHOES —Speed, in knots RW LIGHT RAIN SHOWER CD SCAHERED ECHOES IN AREA RW MODERATE RAIN SHOWER ISOLD ISOLATED FIGURE 22.—Radar summary for 1535 MDT, with locations of fronts and squall line for 1600 MDT, July 31, 1976. 28 FLOOD, JULY Ell—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO FIGURE 23.—Geostationary Operational Environmental Satellite photograph, 1600 MDT, July 31, 1976. Bright areas are clouds. was 0.55 inch, substantially lower than 12 hours earlier. However, precipitable water in the layer from the surface to the 500-millibar level was 0.97 inch, nearly the same as the morning value. Heating and mixing had redistributed the moisture through a thicker layer. The trailing front moved through Denver, Colo., 15—20 minutes after the rawinsonde was released. Therefore, the rawinsonde provided no infor- mation about the extremely moist airmass just a few miles to the northeast. CONDITIONS DURING THE STORM Prior to 1800 MDT, there had been almost no precipitation falling on the foothills in northeastern Colorado. Two to three hours later, catastrophic flooding was occurring. This section describes meteorological conditions along the Front Range from Denver, Colo., to north of Fort Collins, Colo., from 1700 MDT to about 2200 MDT on July 31, 1976. Data were available from the following locations: Fort Collins, Colo. (Colorado State University, At- mospheric Science Building); Greeley, Colo. (Universi- ty of Northern Colorado, Ross Hall); Table Mountain near Boulder, Colo. (National Oceanic and At- mospheric Administration, Environmental Research Laboratories); Rocky Flats plant near Boulder, Colo. (Rockwell International Corp.); Jefferson County and Arapahoe County Airports, Colo. (Federal Aviation Administration); and Stapleton International Airport in Denver, Colo. (National Oceanic and Atmospheric Administration, National Weather Service). Radar reflectivity data were available for the entire storm from the radar operated by the National Weather Ser- vice at Limon, Colo., located about 125 miles southeast of the Big Thompson River area. Reflectivity data for 45 minutes at the beginning of the storm were available from the radar used for the National Hail Research Experiment at Grover, Colo., located about 70 miles east-northeast of the Drake—Estes Park area. PRESSURE, IN MILLIBARS 100 150 200 250 300 400 500 600 700 800 900 1000 METEOROLOGY, HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASIN S 29 / 90 __.t. g/[JEEKOENKO <0 57/55 ~——4_ ‘5“ § \ \ ‘ , Lifted index=—2 \_ \ \ \V\\/ 0,? AK 020? _Tota|sindeX:45 /\ \\ \ \/\\/ \ EXPLANATION <— DEWPOINT TEMPERATURE —— AIR TEMPERATURE —0—— AIR-TEMPERATURE SCALE, IN DEGREES CELSIUS ~303.3— DRY-ADIABAT SCALE, IN DEGREES KELVIN ——10——— MOIST-ADIABAT SCALE, IN DEGREES CELSIUS WIND—DIRECTION AND SPEED OBSERVATION. —Shaft indi- cates wind direction; north is at top. Barbs on Shaft indicate wind speed, in knots. Long barb=10 knots; Short barb=5 knots. FIGURE 24.—Plot of rawinsonde data obtained at Sterling, 0010., 1602 MDT, July 31, 1976. 30 FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO l;5° 110° 105° 100° 95° I I , 30'w\\x ‘ ‘ 30.05 5 30 lo/Zofimuw 0 74 30.16 40° \ 7730.17 86 > 5555 », 94 1 ”v “/3" 61 9312\ ,.,~_« ""’ I 2 9,5 ZFHI‘ 55°: I I} 96 29.92 350 , sgjtm (<30 on 3 329.90 ‘ t 62 10V 1 s ' 23 at? 34 k“ x , .. ”BOX v ~ $9.911 l /60 \ 43.95 110° 100° 0 100 400 MILES | | 41 EXPLANATION 60 65 7O AREA WHERE DEWPOINT TEMPERATURE EXCEEDED L GEOGRAPHIC CENTER OF LOW-PRESSURE SYSTEM 60 DEGREES FAHRENHEIT ”flatly OBSERVATION STATION—Upper left number is air -~——65~——- LINE OFEOUALOEWPOINTTEMPERATURE.—|nterval 57w? \ temperature, in degrees Fahrenheit Lower left 5 degrees Fahrenhen '/ number _is dewpoint temperature, in degrees —-30,15— LINE OF EOUAL ALTIMETER SETTING, lN INCHES OF Fahreflhe't-‘UPPE' ”9th ”“mb9'h's a't'T’te.’ Sign!!! MERCURY.—lnterval (105 inch In Inc es 0 mercury. ower rIg tnurn er IS” _our pressure change, In tenths of a mIIIIbar; -de- _'V—V_ COLD FRONT. crease, ‘\.=increase,and \. =no changes Shaft indi- . . t cates wind direction. Barbs on shaft indicate wind ' STATIONARY FRONT —Oashed where d'ss'pat'"9 speed, in knots. Long barb=10 knots; short barb=5 S OUALL LI N E GEOGRAPHIC CENTER OF HIGH-PRESSURE SYSTEM knots. @=calm winds, less than 2 knots. Additional symbols and notes explained below: R =Thunder heard. but no precipitation at station i’ =Thunderstorm with rain < =Lighting observed G211=Peak wind gust observed during last hour FIGURE 25.—Regiona1 surface analysis, 1800 MDT, July 31, 1976. 40° 35° METEOROLOGY. HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS 31 110° 105° 100° 95° 0 Q "— 0000 I I \ a“ a ‘5 9 ~— 90 I ,, f .2 # LM _ 190 , 40° 40° 35° 35° N0 ECHOES l 100° 200 300 400 MILES l I | EXPLANATION AREA OF RADAR ECHOES.—UsuaIIy interpreted as an TRW+ HEAVY THUNDERSTORM area “"1“ ”'ec'p'mm” TRW+ + VERY HEAVY THUNDERSTORM COLD FRONT TRWx INTENSE THUNDERSTORM —'—.— STAT'ONARY FROM—065““ Where diss‘pating TRW++/+ VERY HEAVY THUNDERSTORM, INCREASING IN —- -— SOUALL LINE INTENSITY TRWU/— THUNDERSTORM 0F UNKNOWN INTENSITY, H GEOGRAPHIC CENTER OF HIGH-PRESSURE SYSTEM APPARENTLY WEAKENING R— LIGHT RAIN |_ GEOGRAPHIC CENTER OF LOW-PRESSURE SYSTEM RW— LIGHT RAIN SHOWER _,10 DIRECTION (ARROW) AND SPEED INUMBERI 0F ECHOES. —Speed, In knms RW MODERATE RAIN SHOWER G) SCATTERED ECHOES IN AREA RW—/+ LIGHT RAIN SHOWER, INCREASING IN INTENSITY 520 LOCATION OF ECHO.—-520=maximum observed 81— RW—/- LIGHT RAIN SHOWER, DECREASING IN INTENSITY / titude Oftop Of echo, in hundreds of feet. 520=52,000 LM RADAR OPERATING IN LIMITED MODE feet TRW MODERATE THUNDERSTORM FIGURE 26.—Radar summary for 1735 MDT, with locations of fronts and squall line for 1800 MDT, July 31, 1976. 32 FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO a . u v u a a. a. n a u . FIGURE 27,—Geostationary Operational Environmental Satellite photograph, 1800 MDT, July 31, 1976. Bright areas are clouds. ANALYSIS OF CONDITIONS IN REGION OF FLASH FLOODING Surface data were recorded almost continually at Stapleton International Airport, Rocky Flats, and Table Mountain. Detailed time series were constructed using these data (figs. 33—35). At Table Mountain, about 6 miles north of Boulder, 0010., the east-west component of the wind was measured from the surface to about 2,000 feet by a Doppler acoustic-echo sounder operated by the Wave Propagation Laboratory of the Environmental Research Laboratories. These time series, hourly observations from the remaining sites, and radar data from Limon, 0010., were used to con- struct the analyses of surface conditions shown in figure 36. Radar echoes for Video Integrator Processor levels 1, 2, and 3 are shown on the maps (fig. 36). These echoes correspond to the minimum detectable signal. 30 and 41 dBZ, respectively. In the Fort Collins, Loveland, and Greeley. Colo., areas, an increase in wind speed and gustiness were the only indications of the passage of the trailing front. The sky over the areas remained partly cloudy during the afternoon with southeasterly winds, resulting in small temperature differences across the front. In the Denver-Boulder, Colo., area, however, afternoon cloudiness was minimal and the resulting heating and mixing had increased the surface-air temperatures and decreased the dewpoints. The winds also were more southerly. The trailing front passed both Stapleton In- ternational Airport and Table Mountain at about 1730 MDT. At these sites, the passage of the front was accompanied by a significant increase in easterly or southeasterly winds. Dewpoint temperatures increased 10°—13°F and air temperatures decreased 10°—12°F in 30 minutes. Prior to 1730 MDT, a large thunderstorm had developed southeast of Denver, 0010., as the trailing front moved into this region (fig. 36A). The thunderstorm moved northwestward and merged with METEOROLOGY, HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASIN S 130° 125° 120° 1 15° 1 10° 105° 100° 95° 90° 85° 80° / 50° 45° \ 40° \ 35° \ 30° \ 25° \ 7 18 I 22 1 00° 0 500 MILES I——I___1_L___1__I EXPLANATION AREA WHERE OEWPOINT TEMPERATURE EXCEEDED 60 —v'—'— COLD FRONT. —Oashed where dissipating DEGREES FAHRENHEIT LINE OF EOUAL DEWPOINT TEMPERATURE. —Interval 5 degrees Fahrenheit —v—-— STATIONARV FRONT. —Oashed where dissipa'ing —- - —- SOUALL LINE LINE OF EOUAL AIR TEMPERATURE. —Interval 5 degrees H GEOGRAPHIC CENTER OF HIGH-PRESSURE SYSTEM Fahrenheit LINE OF EQUAL ATMOSPHERIC PRESSURE AT SEA L GEOGRAPHIC CENTER OF LOW-PRESSURE SYSTEM LEVEL. -12=1012 millibars. Interval 2 miIIibars; 1-millibar interval shown by dashed line FIGURE 28.—Surface analysis, 1800 MDT, July 31, 1976. 33 34 50° 45° 40° 35° 30° \ 25° 125° FLOOD. JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER. COLORADO 20° 115° 110° 1 05° 1 00° 95° 90° 85° 80° \ l \ \ 50° 45° 40° 35° 30° 25° 115° 110° 105° —316— EXPLANATION AREA WHERE DEWPDINT TEMPERATURE WAS WITHIN 6 DEGREES CELSIUS OF AIR TEMPERATURE CONTOUR SHOWING ALTITUDE OF TOO-MILLIBAR SURFACE.—316=3160 meters. Contour interval 20 meters; Ill-meter interval shown by dashed contours. Datum is mean sea level LINE OF EOUAL AIR TEMPERATURE —Interva| 2 degrees Celsius AXIS 0F PRESSURE TROUGH GEOGRAPHIC CENTER OF HIGHEST ALTITUDE 0F TOO-MILLIBAR SURFACE 100° 95° 500 MILES L 10 143 “3‘01 GEOGRAPHIC CENTER OF LOWEST ALTITUDE 0F TOO-MILLIBAR SURFACE OBSERVATION STATION—Upper left number is air temperature, in degrees Celsius. Lower left number is depression of dewpoint temperature below air temperature, in degrees Celsius; X shown when depression greater than 30 degrees Celsius. Upper right number is altitude of 700-millibar surface, in meters; 143=3143 meters. Lower right number is 12-hour altitude change of TOO-millibar surface, in meters; -01=-10 meters. Shaft indicates wind direction. Barbs on shaft indicate wind speed, in knots. Long barb=10 knots; short barb=5 knots. LV in place of shaft and barbs indicates wind speeds were less than 3 knots and direction was variable. M indicates missing data FIGURE 29.—700-mi11ibar analysis, 1800 MDT, July 31, 1976. METEOROLOGY, HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASIN S 35 130° 125° 120° 1 15° 1 10° 105° 100° 95° 90° 85° 80° I | \ I \ T \ {A} \ CM 500 —18 16 +1] 115° 110° 105° L 100° 95° 90° 0 500 MILES l—J—|_|_—1__l EXPLANATION AREAWHERE DEWPOINTTEMPERATUREWAS WITHle —rJ7 537 OBSERVATION STATION—Upper left number is air DEGREES CELSIUS OF AIR TEMPERATURE 3 —01 temperature, in degrees Celsius. Lower left number is depression of dewpoint temperature below air CONTOUR SHOWING ALTITUDE 0F 500-MILLIBAFI SURFACE. — 582=5820 meters. Contour interval 30 meters. Datum is mean sea level LINE OF EQUAL AIR TEMPERATURE. —-Interva| 2 degrees Celsius AXIS 0F PRESSURE TROUGH GEOGRAPHIC CENTER OF HIGHEST ALTITUDE 0F SOD-MILLIBAR SURFACE temperature, in degrees Celsius; X shown when depression greater than 30 degrees Celsius. Upper right number is altitude of SDO-millibar surface, in meters; 581=5810 meters. Lower right number is 12-hour altitude change of 500-millibar surface, in meters; -01=-10 meters. Shaft indicates wind direction. Barbs on shaft indicate wind speed, in knots. Long barb=10 knots; short barb=5 knots. M indicates missing data FIGURE 30.—500-mjllibar analysis, 1800 MDT, July 31, 1976. 36 130° FLOOD, JULY 31—AUGUST 1, 1976. BIG THOMPSON RIVER, COLORADO 125° 120° 115° 110° 105° 100° 95° 90° 85° 80° 50° 45° 40° 35° 30° 25° I I I \ \ \ a} O I :4 V "5 '3' ' / 25° 38 50 '2 54 54 52 5O 48 46 044 ‘ I I I I I \ 115° 110° 105° 100° 95° 90° 0 500 MILES |___|—l__J___l—‘ EXPLANATION AREA WHERE TOTALS INDEX EXCEEDED 50 of? OBSERVATION STATION. —Upper number, where shown, is Totals Index. Lower number, where shown. is Lifted Index, X indicates Totals Index and(orl Lifted Index not determined. E indicates estimated value "—V—r- COLD FRONT —-Dashed where dissipating —'—‘— STATIONARY FRONT—Dashed where dissipating LINE OF EOUAL TOTALS INDEX. —|nterva| 2 units LINE OF EOUAL LIH’ED INDEX‘ —lnterval 2 units FIGURE 31.—Stability analysis, 1800 MDT, July 31, 1976. PRESSUREIN MRUBARS 100 150 200 250 300 400 500 600 700 800 900 1000 METEOROLOGY, HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS \ x \ V \ \ ;X XXX WXW /\ . \ \\ \\\ \\ :\\\\ ‘\: 1” 13°» a a 3 \\ /§ j; W \KE/ »\<\\X$\ \/\ I I I\ ~s>\ ”1% Lifted index— —2 —N \V\ I I Q: _Totals index — 52 E / ,A\y\www*y4 EXPLANATION <—— DEWPOINT TEMPERATURE —-——- AIR TEMPERATURE ~~O——— AIR-TEMPERATURE SCALE, IN DEGREES CELSIUS ——3033-— DRY-ADIABAT SCALE, IN DEGREES KELVIN --10-—~ MOIST-ADIABAT SCALE, IN DEGREES CELSIUS WIND-DIRECTION AND SPEED OBSERVATION—Shaft indi- cates wind direction; north is at top. Barbs on shaft indicate wind speed, in knots. Long barb=10 knots; short barb=5 knots. FIGURE 32.—Plot of rawinsonde data obtained at Denver, Colo., 1800 MDT, July 31, 1976. 38 FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO E >- u I §§ 3030 FP23%2\0 - I I I I E “2‘ a: 1.1. 30.20 - — a: O LLI E 8 30.10 ' I I E 55 '3 E < ,_ 90 1 | j 1 § 30 — _ 5 Surface air temperature C: 70 _. I < LI. U, 60 - — E Mace dewpoint temperature 55 50 — 1 LLI D 40 I I I I I I 1600 1700 1800 1900 2000 2100 2200 2300 TIME, IN HOURS (MDT) EXPLANATION WIND—DIRECTION AND SPEED OBSERVATION—Shaft indi- cates wind direction; north is at top. Barbs on shaft indicate wind speed, in knots. Long barb=10 knots; short barb=5 knots. FIGURE 33.—Time series of surface winds, altimeter setting, and surface-air and dewpoint temperatures, Stapleton International Airport, Denver, 0010., 1600-2300 MDT, July 31, 1976. thunderstorms that had formed ahead of the trailing front in the region west of Denver between 1730 and 1800 MDT. The resulting arc of thunderstorms (fig. 363) moved over the Boulder, Colo., area at about 1830 MDT (fig. 360). The data for the Rocky Flats plant (fig. 34) shows that the passage of the thunderstorms was marked by strong wind gusts and air and dewpoint-temperature changes similar to those occur- ring about 1 hour earlier at Stapleton International Airport and at Table Mountain. Eyewitness accounts and the rawinsonde data from Denver, 0010., at 1800 MDT indicated that the clouds which formed ahead of the trailing front south of Boulder had higher bases than the clouds which developed along the foothills to the north. At about 1830 MDT, a pressure increase of about 1 millibar was observed at the Rocky Flats plant and at Boulder, Colo. This pressure increase was ac- companied by a wind shift to the southwest which in- dicated that rain showers and evaporative cooling in drier air along the foothills south of Boulder had pro- duced a small high-pressure center in that area. The arc of thunderstorms east of Boulder dissipated rapidly after 1830 MDT. The western part of the thunderstorms moved over the foothills southwest of Boulder, but rainfall amounts were much less than observed amounts 20—80 miles to the north. Radar echoes shown in figure 36D indicate the thunderstorms southwest of Boulder were not strong- ly affected by terrain as some of them moved westward almost to the Continental Divide. From the meager data available, it appears that the small high-pressure center developed sufficiently to cause the trailing front to become quasi-stationary between Denver and the foothills south of Boulder, thereby preventing the very moist unstable air from reaching the elevated terrain southwest of Boulder. The winds at Boulder and the Rocky Flats plant remained light southerly to westerly until about 2200 MDT on July 31. METEOROLOGY, HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASIN S 39 Wind observation made at 200 feet above land surface \WIND GUST T0 39 KNOTS WIND GUST T0 41 KNOTS BWIND GUST T0 38 KNOTS Wind observation made at 20 feet above land surface 3 >- .I (D D Z L) I: E 30.30 I | I I | LL] U) _ E 3 30.20 \ M” g E 3010 I I I I I E E <1: : 90 I I E 30 — '-|-' Surface air temperature E 70 < LL 0, so — 3.1" Surface dewpoint temperature 5 50 LLI D 40 I | I I | I 1600 1700 1800 1900 2000 2100 2200 TIME, IN HOURS (MDT) EXPLANATION WIND—DIRECTION AND SPEED OBSERVATION—Shaft indi- cates wind direction; north is at t0p. Barbs on shaft indicate wind speed, in knots. Long barb=10 knots; short barb=5 knots. FIGURE 34.—Time series of winds, altimeter setting, and surface-air and dewpoint temperatures, Rocky Flats plant near Boulder, 0010., 1600—2300 MDT, July 31, 1976. From Boulder northward into southern Wyoming, meterological conditions were drastically different from conditions south of Boulder. The trailing front had moved into the foothills shortly after 1730 MDT and convective clouds rapidly developed. By 1800 MDT, the growing thunderstorms were detectable by radar (fig. 363), and by 1830 MDT several strong thunderstorms were orientated in a north-south line along the foothills (fig. 360). Strong easterly or southeasterly winds and low clouds moving rapidly in- to the foothills were observed by many residents of Fort Collins, Loveland, and Longmont. Cloud bases were estimated to be 7,000—9,000 feet above mean sea level. Surface winds at Fort Collins and Greeley re- mained southeasterly until about 2200 MDT when the wind at Fort Collins shifted to the northwest. The time series of meteorological data at Table Mountain (fig. 35) shows that strong easterly winds persisted after 4; O FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO HEIGHT ABOVE SURFACE, IN METERS 8 o I 90 l I 1 85 — ‘ - w. 80 75 70 35 — DEGREES FAHRENHEIT 55 — ,/-/ {www/ 45— """"" " ,1 (I) c 3' o m In: :. ,_. m 3 'c m -. n: a c -‘ m 5 "\\_ / Surface dewpoint temperature 40 | l 1630 1730 1800 | 1830 I I 1900 1930 TIME IN HOURS (MDT) EXPLANATION “—41—” LINEOFEOUALEASTERLYCOMPONENT 0F WIND SPEED. ——lnterva| 2 meters per second ——+2-~ LINE OF EUUAL WESTERLY COMPO- NENT 0F WIND SPEED—Interval 2 meters per second FIGURE 35.—Time series of winds, from the surface to a height of 2,000 feet, and surface-air and dewpoint temperatures, Table Mountain north of Boulder, 0010., 1630—2030 MDT, July 31, 1976. the trailing front passed about 1730 MDT with a max- imum easterly component of about 48 knots, 2,000 feet above the surface. The easterly surface wind was tem- porarily interrupted about 1900 MDT by a shallow region of westerly winds—probably weak outflow from a large thunderstorm cell located a few miles west of the site. In order to provide an estimate of upper-air condi- tions just prior to the development of the severe thunderstorms, a sounding was interpolated for Loveland, 0010., at 1800 MDT. This sounding, shown in figure 37, was based on rawinsonde data from Ster- ling, Denver, and Grand Junction, 0010., surface obser- vations, and Table Mountain wind data. The sounding data yielded a Lifted Index of —6 and a mean mixing ratio below the frontal inversion of 14.8 g/kg. The lifted condensation level was at the 730-millibar level which agrees with observed low cloud heights at Fort Collins of 7,000—9,000 feet above mean sea level. The data further indicated that the air near the surface re- quired a lift of about 2,300 feet to reach the level of free convection. PHYSICAL MODELS OF THUNDERSTORMS The 10-centimeter radar at Grover, Colo., scanned the storms along the northeastern Colorado foothills until a few minutes after 1900 MDT. Storm intensity peaked about 1845 MDT and then temporarily decreased. A comparison of Limon and Grover radar echoes for this period is shown in figure 38. Limon radar operated at O-degree elevation angle during most of the evening with the center of the radar beam intercepting storms over the Big Thompson area METEOROLOGY, HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASIN S 105° 105“ I ‘11 I. // 'l' _J -------- , -——-—--- 1730 MDT 7500 / , ,0000 1800 MDT Glendevev V Glendevev 10000 A . / 10900 ‘I U ° . 5090 I, 5090 “i I” ' —~”// \\ - - ,5: 'i “““““ e 0/ MIN Kl - ‘§/ / . 1 . tWIndsor .'§'// ‘7 I reeley \V ég, oveland {7 5, / Q” Grand Q, ll 3,9 -nne| OBenne \ . , , \ l" ‘ ‘ \\ 1 / \ // \\ @szth \ ”I \ \ v, / , 3133: Elizabeth \\ 100130 \ 10000 10000 \ v x’ / \ Kin-M \ / \‘ 5‘ ’ “ \ ’l ' ‘\ 6000 I \ \ 000 x \\ I) // Dec’kers . \\ \ Lgrksm" ““18“" \\ ll 7, / arkspm ““35me \ t , / I \ \ \\l I z \ \ 750° 700“ A J ‘r’ 7500 7000 B ' I l0§° 105” / I ______ I _______ ”6000’ 1825 MDT ' 1900 MDT 5090 5000 /'f """"" Nolhns . IWindsor , -Windsur oveland Girl“ , 73000 @450“ OBennel §i k Staplaton '(y , AIrpon , , W n k“ agms IBennet Englo$00d\ u / Airport /,--\ ODenver ’ rkeI Engl Airp ’,_-\ _ (fr-"grin” @\ \ / — , -_, d \ ' PIne’ - Q \ Castle - \ I \ Ban" V Castle - \ ix .Rock Elilaherh. \ / \\ \\ . (1 (A, I ”lock Elizabeth. \ “‘00" \ Kiowa \ 10000 10000 @ w , 7 \ Kim \ 7/ x 6000 / \ ‘3’ \ 6000 \\\ \ ‘\\ .arkswn “\fflhm \\ I, ,7 3rkspur ““33”" \\ \V' i , \\ \ 7000 c ‘1 7500 7000 n I I 0 50 MILES EXPLANATION RADAR ECHO, VIDEO-INTEGRATOH PROCESSOR LEVEL 1 —'-"—— SOUALL LINE RADAR ECHO, VlDEO-INTEGRATOR PROCESSOR LEVEL 2 RADAR ECHO, VlDEO-INTEGRATOR PROCESSOR LEVEL 3 x COLD FRONT. —Dashed where dissipating —v—‘— STATIONARV FRONT. -—Dashed where dissipating WIND-DIRECTION AND SPEED OBSERVATION—Shaft indi- cates wind direction. Barbs on shaft indicate wind speed, in knots. Long barb=10 knots; short barb=5 knots ”flaw—u— TOPOGRAPHIC CONTOUR. —Contour interval, in feet, is variable. Datum is mean sea level FIGURE 36.—Radar and surface analyses at about 30-minute intervals for the Denver—Fort Collins, Colo., area, 1730— 1900 MDT, July 31, 1976: A, 1730 MDT; B, 1800 MDT; C, 1825 MDT; D, 1900 MDT. 42 PRESSUREIN MRUBARS 100 150 200 250 300 400 500 600 700 800 900 1000 FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO / Lifted index = —6 — \ \\\/\\x/(\ V \V \ \ '“ Totals index = 55E / \\ \\ \\\ I I I \V/ I\' I EXPLANATION DEWPOINT TEMPERATURE WIND-DIRECTION AND SPEED OBSERVATION. —Shaft indi- cates wind direction; north is at top. Flag and barbs on shaft AIR TEMPERATURE indicate wind speed, in knots. FIag=50 knots; long barb=10 ASCENT CURVE FOR A PARCEL OF AIR NEAR SURFACE knots; short barb=5 knots AIR-TEMPERATURE SCALE, IN DEGREES CELSIUS LFC LEVEL OF FREE CONVECTION - DRY-ADIABAT SCALE, IN DEGREES KELVIN LC L LIFTED CONDENSATION LEVEL MOIST-ADIABAT SCALE, IN DEGREES CELSIUS FIGURE 37 .—Interpolated plot of rawinsonde data for Loveland, 0010., 1800 MDT, July 31, 1976. METEOROLOGY, HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS 43 at altitudes between 15,000 and 20,000 feet above mean sea level. Grover radar operators varied the angles of their scans, effectively obtaining a three- dimensional record of the storm’s reflectivities. Scans made at 1.9-degree elevation angles intercepted the foothills storms at approximately the same altitudes as the Limon radar and are shown in figure 38. At 1845 MDT Limon radar showed Video Integrator Processor level-3 contours which corresponds to reflec- tivities between 41 and 46 dBZ. Brief periods of level-4 contours were observed later in the evening. Converse- ly, the Grover radar showed a level-5 contour (55—65 dBZ), with a maximum of 64.6 dBZ. The Grover radar data contained more detail than the Limon data because of a l-degree conical beam width as compared to the 2-degree conical beam width of the Limon radar, and because of a much closer loca- tion to the foothills region. The comparative readings suggest that the difference in beam width of the two radars enabled the Grover radar to detect small in- tense cells that were averaged out in the Limon radar signal. This difference appears to be about 15 dBZ in the center of the thunderstorms. A two-dimensional cross section through one of the largest thunderstorms was constructed, as shown in figure 39. This thunderstorm became quasi-stationary approximately 1845 MDT near Storm Mountain, which is about 5 miles north of Drake. The cross sec- tion is positioned in a line from southeast to northwest, approximately along the direction of low-level inflow. Grover reflectivity data, visual observation of cloud formation and movement, satellite observation of the areal extent of the cirrus anvil, and the interpolated sounding for Loveland were combined to give a schematic but fairly detailed picture of the storm structure. The strong inflow allowed a large amount of mass to be processed by the storm. As the low-level flow ap- proached the Front Range, a shallow layer of stratus and stratocumulus clouds formed in the layer between the lifted condensation level and the level of free con- vection. A surface observation at 1800 MDT at Fort Collins indicated a thin broken deck of clouds based at 4,000 feet above the surface. When the low-level air was forced above the level of free convection, explosive convective growth occurred. The radar data indicated that new cells formed in the inflow and moved north- northwestward into the storm. Over the mountains, the cloud base was effectively on the ground; the high in-cloud freezing level was at about 19,000 feet above mean sea level, and the height of the —25°C isotherm was at 31,500 feet above mean sea level. This indicated an unusually deep layer for warm cloud condensation and coalescence processes to act. There was weak wind shear above the level of free convection; therefore, little entrainment of drier middle- and upper-level air into the storm. With the cloud base on or near the surface, precipitation was falling with virtually no subcloud evaporation. Neither entrainment nor evaporative processes were able to produce strong downdrafts within the storm, thus yielding a highly efficient storm. Grover radar data in- dicated that the storms which were moving into the foothills generally sloped to the east or southeast. Once the storm became quasi-stationary over the elevated terrain, they tended to slope to the northwest as shown in figure 39. The northwest slope of the up- draft allowed large precipitation droplets to form and then to fall out of the rear of the updraft. This enabled the system to exist in a nearly steady state. Efficient unloading of the updraft in the lower half of the cloud permitted large updraft velocities to develop within the ice phase upper cloud, which, in turn, pushed the cloud top to very high levels. Indeed, radar observa- tions indicated that the maximum tops of the thunderstorms were about 62,000 feet above mean sea level, or about 6,000 feet higher than the tops of any other similarly reported thunderstorms on the eastern slopes and plains of Colorado. Some 20 miles to the south of the storm portrayed in figure 39, or about 5 miles southwest of Lyons, another storm of similar size and intensity developed between 1800 and 1845 MDT. A sequence of outstanding photographs of the development of this storm was taken by Mr. John Asztalos, who was located at Mitchell Lake approximately 15 miles west-southwest of the developing thunderstorm. These photographs are shown in figure 40. Note the similarities between the photographs and the cloud model in figure 39. RAINFALL ANALYSIS The total rainfall for July 31—August 2, 1976, is shown on the isohyetal map (fig. 41). The analysis was based on rainfall records at stations in the National Weather Service climatological network and rainfall reports from 119 miscellaneous locations in the storm area. Unfortunately, the lack of detailed rainfall- intensity data and the inaccessibility of the area over which the storm occurred resulted in data depicting 44 FLOOD. JULY 31-AUGUST 1. 1976, BIG THOMPSON RIVER, COLORADO EXPLANATION ——2—— RADAR ECHO OB- SERVED AT LIMON —Video Integrator , Processor level ——15— RADAR ECHO OB- SERVED AT GRO- FIGURE 38.—Comparison of ra- VER—Number x: rel» dar echoes observed at Limon fu‘rfidpflfgrg; 'ien and Grover, Colo. dBZ. Areas with val- ues greater than 55 dBZ are shaded METEOROLOGY, HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASIN S DISTANCE FROM STORM MOUNTAIN, IN MILES 20 IO 0 IO 20 30 ALTITUDE ABOVE MEAN SEA LEVEL, IN THOUSANDS OF METERS I I | I I I 5" 18— —55 17— 16— _50 15— T f 14—— —45 f 13— _40 12— °\7 11— —35 m— (3 430 e- x 3“ —25 7.. f n —20 6— J I 5— : f 15 4O 3O 20 IO 0 IO 20 3O 4O 50 DISTANCE FROM STORM MOUNTAIN, IN KILOMETERS EXPLANATION (-—— SCHEMATIC LINES 0F AIRFLOW —LFC— LEVEL OF FREE CONVECTION | | I I I | | | | I | SCHEMATIC AREA OF RAINFALL —LCL— LIFI'ED CONDENSATION LEVEL —/5— RADAR REFLECTIONS OBSERVED AT CROVER, COLD—Dashed WIND—DIRECTION AND SPEED OBSERVATION-Shaft indi- where approximately located. Interval 10 I132 f cates wind direction; north Is at top. Barbs on shaft Indicate wind speed,in knots. Flag = 50 knots; long barb =10 knots; —O°—- LINE OF EQUAL AIR TEMPERATURE, IN DEGREES CELSIUS.— short barh=5 knots Dashed within the cloud FIGURE 39.—Physica1 model of the thunderstorms over the Big Thompson River area at 1845 MDT, July 31. 1976. ALTITUDE ABOVE MEAN SEA LEVEL, IN THOUSANDS OF FEET 46 FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO FIGURE 40.—Large thunderstorm several miles southwest of Lyons, 0010., at about 1830 MDT, July 31, 1976. Photographs taken at Mitchell Lake looking east-northeast. only the 3-day rainfall totals. A tabulation of daily rainfalls for July 31—August 2, 1976, at 10 National Weather Service stations is shown in table 1, and total rainfall amounts for the same period at the 119 miscellaneous sites are shown in table 2. The precipita- tion summaries in tables 1 and 2 and the isohyetal map (fig.41) were prepared by the National Weather Ser- vice, Central Regional Headquarters, in cooperation with other Federal agencies. For most of the area, the analysis in figure 41 pro- vides an overestimation of the actual precipitation which produced the flash flooding. Also, the analysis offers little evidence as to rainfall intensities associated with the severe flooding during the night of July 31—August 1. Two continuous rainfall records obtained in the Bellvue area northwest of Fort Collins and the record obtained at Allenspark (fig. 42) provide a good TABLE 1.—Daily precipitation, in inches, Boulder and Larimer Counties, Colorado [T, trace; leaders indicates no data available] Location Time of 1976 Station Latitude Longitude Obifi‘ggon ____.;\111y ——_—1 August 2 Allenspark .................... 40° 12’ 105°32’ 1700 0.02 0.50 0.90 Boulder ....................... 40°00’ 105°16’ 1700 T .12 .69 Boulder 2 ...................... 40 ° 01 ’ 105 ° 16’ Continuous 0 0 Estes Park .................... 40°23’ 105°31’ 1600 T 3.59 .87 Fort Collins ................... 40°35’ 105°05’ 1900 .23 .11 .50 Fort Collins 9NW .............. 40 °40’ 105 ° 13’ Continuous 3.1 2.2 1 .8 Longmont 6NW ............... 40° 15’ 105 °09’ Continuous 0 0 .7 Nederland 2NNE .............. 39°59’ 105°30’ 0800 0 .19 .73 Red Feather Lakes 2SE ......... 40°48’ 105°34’ 1700 1.20 .33 .12 Waterdale ..................... 40°25’ 105°12’ 0800 0 .33 .46 METEOROLOGY, HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASIN S FIGURE 41.—Total rainfall in inches for July 31—August 2, 1976, along the Front Range in northeastern Colorado. .__8_ 0.0.0..... 7— --6000--- EXPLANATION LINE OF EQUAL CUM- ULATIVE RAINFALL —Interval 2 inches DRAINAGE DIVIDE DRAINAGE TOPOGRAPHIC CON- TOUR—Contour in- terval 2,000 feet. Datum is mean sea level 105°oo' 47 48 FLOOD. JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO TABLE 2.—Total rainfall, in mches, for July 31 —A ugust 2, 1976 at TABLE 2,—Total rainfall, in inches, for July 31 —A ugust 2, 1976 at miscellaneous sites miscellaneous Sites—Continued [From National Oceanic and Atmospheric Administration, August 1976] Location Location Town- Total . Observer Town- Total . Observer 5:111}! “$339 Section Quarter rainfall Type Rating and remarks ship Ravage Section Quarter rainfall Type Ratmg and remarks Boulder County Larimer County—Continued 3 71 1 1 C 1 6-8 Gaze Good George Hircock- 6 72 34 sw 2 7.2 Wheel Poor Cliff Manson. 3 71 12 C 25.0 ....... Fair Verle Bradshaw. 2 barrow 3 '71 13 NW 35 P01: Fair 0 13- Sisk—heaviest 6 73 11 SW 12.0 Can Poor Michael Davidson. between 2100 and 2300 MDT. July 7 70 5 6.4 .............. Soderberg. 3}; 7 70 5 9.1 Gage Good Steve Cox. Colorado 3 72 4 SW 4 3 Gage Good Philip 'l‘revarton. State University. 3 72 34 SW 3.0 Bucket Fair Nejl Turner. 7 7o 12 NW 7.0 Bucket Fair Joel Lamoreux. 3 73 12 N 2.9 .............. W111 Waite. 7 71 14 NW 5.0 ....... Fair George Mornik. 3 73 13 SW 1.0 -------------- Clayton Ward; 7 71 24 SE 14.5 Bucket Fair Jay Howlett. 3 73 23 1.0 Can Fair Dan R. Cox. 3 73 25 1~12 Gage Good W. A- Renae 7 71 14 4.5 .............. G. Marwick. 7 72 12 10 .............. Tom Smith. Larimer County 7 72 13 8 .............. Bonnie Carter. . 7 72 15 SW 6 .............. L. Rogers. 4 71 29 SE 435 Gage 600d Dottie Branurn. 7 72 15 2.3 .............. Helen Heather- 4 71 32 2 3.75 Gage Fair Lillie Olander. ington. 4 72 2 SW 9.0 Pot ....... Fred Hurt. 4 72 5 C 6.0 'I‘ub Fan Houck. 7 72 17 4.25 .............. Helen Dickerson. 4 72 7 6.62 Bucket ....... D. A. Carvell— 7 73 16 SW .68 .............. T. Noonan. heaviest between 8 69 30 SW 4.6 Gage Good Harold Craw. 2030 and 2100 s 70 15 7.8 .............. Unknown. MDT. July 31- s 70 29 11 25 .............. John Park. 4 72 11 C Bucket Fair Byron 'l‘udder- 8 7o 29 sw 9.0 .............. John Baker. 4 72 35 C ~ - . Fair Bob Swape. 8 7o 30 4.0 .............. Mary Williams. 4 73 1 NE ------- Georg: Baced- s 70 31 NE 10.24 Gage ....... Pete Wetzel— 4 73 3 SE ....... Ken cDowell. recording rain 4 73 14 SW ....... Gordon Mace. gage. (See fig. 1.) 8 71 1 SW 9.0 .............. Archie Lan ston— 4 73 23 SW ....... Robert Irvin. heaviest tween 4 73 34 NW ....... US. Forest Ranger. 2200 and 2400 5 69 34 NW Good Darrell Fargo. 2 MDT, July 31. 5 70 9 C ....... Clemont Young. 3 71 5 5 .............. Kelvin Danielson. 5 70 8 EV: Fair Unknown. 8 71 20 9.0 .............. G. Garrison. 5 70 28 NE 600d Unknown. 8 71 20 C 5.5 Can Fair Leon Ferguson. 8 71 23 SE 7.5 Tub Good Wlmmer. 5 71 2 NW 3.3 ....... Fair Unknovm. s 71 29 15.0 .............. J. Veen. 5 71 3 NV: 35 Gaze Good Unknown. 8 71 32 SE 7.5 Bucket Fair Dan Colter—heavy 5 71 16 NE 2.0 Gage ....... Monte Christman. intermittent rain 5 72 4 SE 6.0 .............. Julius Hamilton. 1600—2400 MDT. July 31. 5 72 6 C 6.0 ....... Poor Frank McGraw. 5 72 8 SE 1 28.75 Bucket ....... E. L. Neuswanger. 8 71 35 11.25 .............. Unknown. 5 72 8 NW 1 26.0 Bucket Poor Yam; 8 71 36 7 .............. Bill Cotton—heavi- 5 72 10 NW 6.0 Bucket Poor _ Godesiabois. est between 2315 5 72 12 SE 10.1 Can Poor Unknown. and 2345 MDT. Jul 31. 5 72 18 SE 14.0 Gage Fair 0. H. Woods. 8 72 11 26.5 .............. Kelvill Danielson. 5 72 21 NE 11.0 Bucket Fair Geor e Guthrie. 8 72 25 NE 7.0 Gage Fair Ra Vannorsdel. 5 72 22 NE 11.5 . . . . . Fair Haro d 'l‘regent. 8 73 14 NE 1.50 .............. T. illiams. 5 72 22 NE 9.7 Fair Ben Federson. 5 72 23 NE 10.0 Poor David Ruhn. 8 73 25 ....... James Noowan. 8 75 11 ....... Unknown. 6 72 28 SW 10.75 Bucket Poor Harold Shipper. 9 69 1 1 ,,,,,,, Unknown. 5 72 29 sw 5.75 .............. Hertzler. 9 59 23 NE 1 ....... H. A. Simpson. 5 72 30 NE 5.3 ....... Good US. Forest Service. 9 7o 12 Ev, Poor Unknown 5 72 30 4.66 .............. Estes Power Plant. 5 72 34 NE 10.8 Gage Good Michael WappriCh. 9 70 33 ....... Terry Van Cleave— heaviest between 5 73 15 SW 1.03 .............. James Work. 2200 MDT, July 5 73 22 SE 1 1.25 .............. Les Casswell. 31, to 0200 MDT. 5 73 23 SW 3.8 ....... Good Michael Marden. 2 Aug. 1. 5 73 26 2.60 Gage Good Howard Karp. 9 71 19 C 7.0 Can Poor Unknown. 5 73 31 C .38 Gage Good Walter Hines 10 70 15 4.26 .............. Clarence Koch—. heaviest around 5 73 34 .77 .............. National Park Ser- 2300 MDT. vice. July 31. 6 70 9 6.0 .............. Ed Dion. 1o 70 32 4.0 .............. Unknown. 6 70 9 2 1.1 . . . Kelvin Danielson. 6 71 1 NE 50 Ra Berg. 10 7o 34 6.25 ....... Ed Nauta. 6 71 6 7.5 U own. 10 71 9 6.2 ....... .. D. H. Webb. 1 10 71 36 6.0 ....... . . Tom Thomas. 6 71 27 NW 50 Ray Ber - 1o 73 34 .6 ....... US. Forest Service 6 71 27 4.1 . Ed Smit . Ranger Station. 6 71 34 SE 4.1 Lee Kriebaum. ll 68 13 5% 1.9 Gage Chuck Roberts. 6 72 16 SW 2.0 Don Chelle . 6 72 23 SE 12.0 Bucket Good Frank Fai a. 11 69 31 1. Graham 11 69 31 Unknown. 1 1 71 15 SE Bill Logan. 6 72 25 NW 8.0 Bucket Fair Unknown. 12 70 20 .. W. R. Mordah. 6 72 27 SW 12.0 Bucket Fair Clarence Nold. 12 71 31 SE . F. Main. 6 72 27 NW 12.0 Bucket Poor Gordon Leonard. 6 72 27 SE 29.5 Bucket Fair Unknown. Albany County. Wyoming 6 72 27 SW 7.5 Gage Poor Leonard Ray. 2 13 72 24 4.8 .............. Lois Bath. 6 72 27 NW 4.5 Can Poor Joe Bauer. 6 72 28 2 8.0 .............. Barbara Havens. :July 31 evening total only. 6 72 28 SE 10.5 Tub Poor Warren Cross. Rainfall total was greater than amount indicated. METEOROLOGY, HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASIN S 49 12 I I I T l Locta I ' I l I I ion in SE%NW% Sec. 31, T. 8N. R. 70W., Larimer County Total=10. 24 inches (260 millimeters) I 10 4 ._ -" A" ’* I I I f- I I II I I I l l I Ft. Collins, Larimer County— I I I I l I — r‘I— l l l I Total=7.9 inches (201 millimeters) I I I J I f '— I I / I I I l I i ~_ CUMULATIVE RAINFALL, IN INCHES I I I i I I I I I I I I I I I I I I I I I I I I I I I I I I I l I I I I 0 II I l Allenspark, Boulder County I I Total=1. 4 inches (36 millimeters): l I I X-» I"‘L"I’I 'I/f. —-l I I I I I 1200 JULY 31 2400 2400 1200 AUG. 1 2400 1200 2400 AUG. 2 TIME, IN HOURS (MDT) FIGURE 42.—Cumulative rainfall, at three stations in Boulder and Larimer Counties, 0010., July 31— August 2, 1976. representation of rainfall distribution during the storm period for those areas. In the Big Thompson River basin, no continuous rainfall records were available but radar data and eyewitness accounts provide a fair description of the storm period. The North Fork Cache la Poudre River basin in the vicinity of Virginia Dale lies outside the range of the Limon radar; thus, the timing of rainfall in that area is based entirely upon eyewitness accounts. Limon radar-image locations with relative inten- sities are shown in figure 43 from 1701 to 2200 MDT on July 31, 197 6. The images are shown at 20-minute intervals prior to 1900 MDT and at 10-minute inter- vals thereafter. The first thunderstorm cells developed between 1800 and 1830 MDT several miles east of the maximum rainfall zone near Glen Comfort and Glen Haven. The storms were moving generally north- northwestward and reached a temporary peak in inten- sity around 1845 MDT. Between 1900 and 1930 MDT, the individual cells tended to merge, and the rainfall pattern continued to shift slightly westward. By 1930 MDT the most intense rainfalls were southwest of Drake. From about 1930 MDT until shortly after 2100 MDT, the “cloudburst” phase of the storm occurred in the Big Thompson River basin around Glen Comfort. The storm complex continued to shift very slowly to the northwest, and after 2100 MDT, the most intense 50 FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER. COLORADO ‘05“ “5° // | _______,1 ll // T _____ ,1 :/6000 1701 MDT 7500 ,//5000' 1720 MDT / ‘ // / Glendevev \ ,’ // a \y/ ‘7 10000 n ,4! \ 5000 \ 1 1 “5‘,“ ’ ‘1 5000 ,,~/” \ 7 ,~ 9°» \‘\ i x'” ____________ 7’ 1 7 \'>/ \ \ ’,-_____,,_// . x _ \\ 037/“ 1‘ 1 . / Ft. Collins \ w K , \ ,3 Ft. Collins ' . \G/ “’/ Glen 1 ,r l _ \ OWIndsor o/ ‘ OWindsor . '1 $3 / / v 1 ’01 QLoveland 'G'EE'" g7 ” oveland 'G'Ee'e" 4g? ( Estes Cnmlort‘i \ q9°, \ ll Gland \\‘ Park // /' Bertha-Lid / Grand ‘\ , La.ke ‘, [7 1’ \I ll Lake ‘1 I'Me'sker '\\ \ / \ / ’ :Park \\ 11 .Tahle 1' l?“ \ '1 .Table ,1 Siam \W.ard / ,l M" L ,—\ Granhv LwOam (,7' II ""- k‘ ,0 ,’ Boulder ,' I Boulder / \\ I0 H \ in ~ ‘0“ — ll/lederlznd 1 '1‘ 40” nedeylgna \‘1 \ oarighion \\ ’1‘ \\ \\ Jellcu '\ \\ ‘1‘ Jerrco O 501 ‘\ Rn'ck \‘ ' R .k ,7 Central I Flats" Stapletun “/7 Central / Fo'c‘y ' Ciw ‘7' \ Airport W /, _ c" 7' ‘a s ' .\Golden ’ ’ \G Id nEmnne '\ \ 'Denver. I 'Em'm '\ 'l n an OD -A 1 ' - \ ' 7 1 / /—. \ 0 a 0,: u ‘ x 1 1 Englwvuo N En Iewund ‘\ Evergreen\ A090" ”_\\\ ’ Eirport /__\ // \ , /' ,e-"iParker \ C ,»-—’iParker I 7’ \' \ Sedailra \\\ " \\ $20 /I' \ / E, Bailey ‘ \n; \ \\ Emmy Pine”" \\\ ”a re V \ I \ . - \ — Ca - 11m \ \ x . y \ v Castl - h \ ,/ \ \ oRonk @a e \ I], \ \\ 7’ /,-) r‘\\ .Rock 1121 \\ 10000 10000 — ’ ~ ~ \ Kio'wa \ 10000 10000 \ *—’ ,7 [I \ ‘~—\ Kio'wa \ ’/ \\ ‘\ 5000 / \\ 7‘) I/lzeckers \ \\\ 6000 \\ 7 \ \EK \ ‘ \\ Lg'ks'w' .‘Eibm \\ 1" I" // \\ Larkspur 7\“~.\E'hm \\ \ \ \V' l // \ \\ 7500 7000 " 7500 7000 I l 105° 105° ‘1 // l _______ ’1 i // I ______ J 7500 ,//5000 1750 MDT ’/6000 1800 MDT '1 / / ,/ / Glendevev '\ , Glendevey u 'y/ a n I 7 10000 ust’rc 7‘ 5000 1‘ U 5000 F 9‘) ‘\ //’ 1 ' /,~ 7 0‘7 1 // \ ,/ .'~/l ‘1 r """"""" 1 / """""""" , . \ § , ‘ \ . , . ‘\_,_ \ v t. Collins \ 0/ \ Ft, Colllns /Glen 1 , 1‘ W d 97 1‘ w d n so a n sor / H""""/D.rake l "' r 15/ '1 ' / ‘ 1 \~ , 1 1 / lb Ian 1' P oveland 'G'ee'“ g/ f . V{men 1' QLoveland 'G'EE'EV ’ E5711: Comfonl \ 755/ 1' Estes Comforl‘i \ ‘\ Park 7' / Berthuud ll Grand \\ Pavk / / Bertha-rid “, 1’ 7’ ' \ , Lake “‘ / / ‘ 7 . l ' ,' CD ' 7 . l '1 «Meeker l \\ // I\Meeker \\ \\ \‘Park \‘ 1' .Tlahble ‘, \IPark \1 .2 1 , - 1 r. Granny \lwfm 7" '1 l 'k/ \ 7/ \\\ Granbv \‘1w'a'd /7 : / 1‘ Boglder \J ‘--\\ / 1‘ Bo‘older \ \ _ 1, x W’— Medal-and \\ \ OBrighrun \\\\ 40 — liederlgnd \\ \ H l \ ’ M l \ K L‘ \ . _ A 0 5000 \\ ix \ \ ,/' Central 7' R / ocny / IBenner , OEmpire 1' . Englewuod Engl 0d Airport /.. £07er \ l/fi’fiarker \ Q ,J \ ,. . :\Sed,alra ca I O \ \ ‘w :1 e v \ / Castle - ’/ \ ofiock Elizabeth. \\ // nRock Elizaherh. \\ 10000 \~\\ Kiowa \ 10000 Kiowa \ / \ 0000 / \ 6000 \\ Larkspur 7\\‘\*:\E'bm \\ Lzrkspur \7“~:\E'hm \ \ \ \ \ \ \ \ 7500 7000 7500 7000 l l 0 50 MILES l’4__1____1—A__J EXPLANATION RADAR ECHO, VIDEO-INTEGRATOR PROCESSOR LEVEL 1 RADAR ECHO, VIDEO-INTEGRATOR PROCESSOR LEVEL 3 --—-6000---— TOPOGRAPHIC CONTOUR. —Contour interval, in feet, is variable. Datum is mean sea level FIGURE 43.—Radar echoes in the Denver-Fort Collins, 0010., area, observed at Limon, Colo.. from 1701 to 2200 MDT, July 31, 1976. METEOROLOGY, HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASIN S 51 105° II x 1 ______________ 7590 //:/6000 1820 MDT ‘ 1840 MDT \ , / Gl'endevey ‘\‘ /,/ [1/ Glendevey y | 10000 . // moon /1 / v ‘x U “:59“ { soon \ .1”; 5000 \ l \>/ ,——” \ ,, / \ "M 1 \ - “““ l ' ------- \g’)’ ‘\-_-.)G|en\l / Ft.‘Co|lins \ . _\é/ / Havem' 1 OWinnsur °Windsor .\§/ / '/ ‘ 1 .§' §,/ / . {i {Loveland 'G'EE'" g7 oveland 'G'EE‘BV E Q/ \\ Esm ‘30 \ (fl / Grand \ rk arrthud // Gland / W ‘1 ‘ Luke I, I I . [\Meekar | )Park Granny Kw.“ Granby /' '\ \_/ “»—-\ W‘ Rederlfnd \ 40’— OBriggn l 1 . \(5 Id ‘ Warkms -‘ a an ODenver . IBennet Englevznad ’,_-‘ Airport \ ,/ \\ rker \\ Castle v Castle - . Rock El'uzaheth. // . Ruck Euzabeth. \ Kiowa 10000 Kiowa \ \ souo / \ snuo \ \\ ‘\ \ L§vkspur ‘~— \\ Lgrkspm »E‘\-\E|hert \ \ \ \ \ \ 7500 7500 7000 | | 1E6” 105“ 1 r ' ........ 1 / ' _______ 7590 /’/euon 1900 MDT 7590 ,x’,,snon‘ 1910 MDT / / / | / 1 / / Glendevev ‘\ ,y/ // Glendevev ‘\‘ /,’ I’/ 10000 >' ‘ f1 1“ w 5090 509“ \\ [1'09 ———————————— ,x' ____________ ,x‘ \\ 1‘.) . // ‘ . // . x W Ft. Collins t COHIDS 0/ r 1 _\/ \ OWindsor ‘. OWindsur $7 1 1 3/ , {Loveland 'G'EE'EV {Loveland careelev / \ \ 7/ Grand \ Barthu'ud Bevthaud ake ‘ ' \ I . ) \’ ll ‘ // .Tahle ‘1 1 .Tahle ‘1 Granbv ML \\ _ Ml. l.\ v \ V"\ I \ der \ , \ . r \ ,I ‘-—-~\ - - \ ‘0" - oarigmun .\\ ‘ uBrighmn ‘\\ - Julian soon Julian 51100 1 . . I Rocky ncky Central I Suplemn Staplamn 'Cirv (l Flals Airport 1'] Flars Airport - "5 oasnner \Gnlden ' Watkins B t , 'Denver ' ', _ ODenver , . enne Ennleu7 Alma - —\ / x \ \ / \ ,,— Parker \ / -0) \ Segalia a 1 my“; \ Sega/Ha \ \\\,/ 3.9 I \ ‘J / \ A‘x‘ ,‘fij‘u'f amber? / -V ,3 1 ,\\\ 3?": Elizabeth ’ 1 1/ \\ ' .I \ ’ . ’ //I /’ \\\ Dc ' a ‘ 10000 / \ ~~\\ mea \ moon w ‘— / /, \ \_\ Kiawa \ / IDeckers \ \\ 6000 / 7‘) /Deckers \ \\ 5000 , \ \\ I - l \ \\ \\\ Larkspur “~—:\E|be" / 1/ ’/ \ Lzrkspur \ \ \ I / \ \ \ I I \ 7500 7000 V 7500 I 1 0 50 MILES 1—I—A—l____l__~_l EXPLANATION ————eoao---- RADAR ECHO, VIDEO-INTEGRATOR PROCESSOR LEVEL 1 RADAR ECHO, VIDEO-INTEGRATOR PROCESSOR LEVEL 3 TOPOGRAPHIC CONTOUR. —Contour interval, in feet, is variable. Datum is mean sea level FIGURE 43.—Continued. 52 FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER. COLORADO 1920 MDT 5900 5090 OWindsoi IGrzeley oveland queelzy \_7_ ’- \\ If “ L \_/ ‘_‘\\ 'Biighton \\L\ _ 5000 Staplemn Staplewn Aryan W k’ Airport at ms 502""!!! Watkins ODenver ' ODenver , 'Blllrlzl Englefioud {1” Englevzuod Airport l,..\ \ EWW'EB" Airport ,_-\ x, \\ \ \ //”’3Parker \\ ' \ /,»—» oParker \\ Sed’r \ \ ~ \ \“w‘lala Castle . \ \ Bail” Pines/“l ‘l‘ieg’aha Castle \ I \\ Ifiock Elizabeth \\ l/ \ \‘ . 4’1/7‘ ,AX aRnizk Ellxaheth \‘ l \ ‘»—~\ diam iDDOU 10000 ‘\ \—’ 1/ [I \‘\~-\ Kio‘wa \ \ \\ 6000 / \\ ‘) l/Deckeis \ \ 0000 ~\ / ’ ‘\ \\ L3'“‘”“' \‘flhm \\ {I / z" ‘\ szkspm \‘x'flbm \ \ \ J i I \\ \ 7500 7000 ‘ V/ 7500 7000 | l “56 105“ ‘l / T ________ 'I I I ________ 7500 / /6000 1941 MDT 7500 / ,,6000' 1950 MDT I , ‘ / Glendevey \\ I Glendevey ‘\‘ /,/ 1/ y | 10000 10000 I/ \ ,n r. ' // \ \\ U 5000 ‘* R253)” ‘i 5000 ‘ /,/’ 0/ . /,/” \‘\ ',\ \\ o (”i —————————— \‘ o x’ri —————————— \g; [,1 t. olllns I Ft.|Co|IIns “57/ e \ IWindsor \ OWindsor .§/ I I \\ l i 5/ {Loveland 'Gmm 0 L0 'eiand ’G'BE'EV / \ x ‘7 Grand Berthuud Benhoud , ake ' \\ . X . Ii ; . ‘ \ , \ l / ‘|.Tnble f .0012 / Gianbv a I) : Ml. \_/-\ /,—\ ML \ _\ f‘ : Bo‘glder \J ‘ ‘‘‘‘‘ \ Bolqlder \J 40"— Redeiland \ ‘ 'Bvightun IBnghmn _ mica Jeilcu ! RV Sta nkv pietun Sm lawn ‘ ais Aiipnn w I‘m Aigpun o\Gulden ' atkins .Beflng‘ “Golden ' Watkins H mm \ ODenver ' \ ODenver ° . a \ \ ’ _ \\ Englefiuod (4’ } \\ Engiwzood \\ Airport ’,_~\\ Evargreem\ \ Airport ’,_-\ / \ / ‘\ A , \ _./ \ \\ ‘\ SM, IParkev \\ \Y \ /, OParker \\ / . __z \ e aia \ x . __, \ Seda|ia \ / \ Bailey PI'EE’ ‘ \\ / \ Bailey Furie’ \\\_0, \ I/ \ \\- I; f) ‘ $33: Elizabeth \ // \ \\- I} /\ {‘0‘ .00?ng Elizabeth \ 10000 10000 \\ ‘—’ —\ Km'wa \ 10000 10000 v 1/ / \\ —\ Kio‘wa \ / \\ \\ 6000 / \ “‘2 l/geckérs \ \ 6000 \ \\ \ I \ ‘\\ \ \\ szkspur “““ . jibe" \\\ (I II / \\ Lsrkspnr \‘~:\Eiben \\ ‘ \\\J' i ,Il \ ‘ 7500 7000 " 7500 7000 | | g 50 MILES |—4;_‘i__.l—l—I —---6000---- EXPLANATION FIGURE 43.—Continued. RADAR ECHO, VIDEO-INTEGRATOR PROCESSOR LEVEL 1 RADAR ECHO, VIDEO-INTEGRATOR PROCESSOR LEVEL 3 TOPOGRAPHIC CONTOUR. —Contour interval, in feet, is variable. Datum is mean sea level METEOROLOGY, HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS 113° 106” I 1 | ______,7 7' 1’ 'I' ______ 7500 ,/,,0000‘ 2000 MDT 7500 / ,6000‘ 2010 MDT ' / . // /, ‘ / Glandevev \\/,/ / Glandevev V 10000 . ”Y 10000 n // 1 \ {.5 “I," 5090 ‘1‘ C 5090 \\ - ,—// \\ - —"/ 1 7 1 ,a ———————————— ’ \\ 9‘; F- / . \\ 0/ / ' ‘0, t.‘Colhns \ 007/ Ft.|C0|lIns _ \/ ‘, IWindsnr o‘,’ ‘, IWindsur * l “($7 1 I ' I gLoveland aGreelev ‘9.” {Loveland lG'BeIEy / \ d 7 Grand Berthoyd \ Lake ‘ 1 / . \ l 7/ 7 Jam ,1 ,_\ l/‘\ Granhv ML \- _\ /r\ \/ ulder \J \7—\\ IBrighmn ‘ 40“- IBrighron \‘~\\\ " \ Jeflcu 5000 I \I . Smplemn - antral 7‘ “01:3 Staplamn ALrpon W lk' City 7/ ‘5 ' ALrpun W k‘ a ms IBannel 1 -\ 0000 II M aBenner ODenver ‘ \\ \ 'Denver ' 7 Englw’nnd {4"} ‘1‘ Euglefiaud Airpun ,--\ Eve nreem\ Airport ,__\ / x ,r \ \ /"";Parkar \\ \ \ //‘-’5Parkev \\ \ \ \ . __‘ \ Sedaha \ \ . -_' \ Sedan: \ / \ Bail'ev Pma’ ’ fix.” me El b h \\ ‘ \ Bailgv Pyle” ’ {N3 Cam \ \ 7 - . \ / 7 1006; Comm \\ {1/ I 7’A\\\\ onock .Iza 2t.- \\ 1/ 13000 ( /,~\/ 7’A\\ \ 0R00k Elyabal-h. \\ \ \\ ,1 (1 -~ Klnwa \ 10000 \ \ — I (1 \\ \\ Klowa \ / \\ ‘2 (IIEBEKEIS \ \\ 6000 2’ \ “) (Illaeckers \ \\ 6000 7 \\ \ 1 \\ \\ '1’ III I/ \\ Lzlkspur “~ 3511"" \\ I/ ,I/ /’ \\ LErkspur “ Lab?" \V' I ’ \ \ ‘\) .' / \ \ ‘/ 7500 7000 V 7500 7000 | I 105° 105° / _____,I II // | _______ ,///6000 2020 MDT 7500 / ,Auoo‘ 2032 MDT / ‘ / Blendevey \ ,/ // u \y/ ‘7 10000 10000 . - ” ‘1 ,1 1/ 1 5000 \\ U Rush: 1: 4" \ - ,/ 1 <9 1 ,- __________ / 1‘ {a}. \ ,. ————— ’ . \ «9' \‘ ’ . .|Col|1ns \ 68/ ‘ . FL‘COIIIHS .Windsur $>/ 81(8 1“ owindsnr '\\ _ 1 1 LoveIand 'G'EEIEV «5/ ,1/ - I: '\ Loveland IGreeIEV / KI \ 2” 4" Grand ‘ ’ Benhuud / L k / ' \ I 1 / 0.9 \ I // / I. .Tablz II _ Granbv , ML 1 _ F\ , o 7’ 1 ’ \ l \ I\ Boylglder \J ‘~-\\ W‘- — 40°’— 0 \ \ -Brighmn ‘\\C ‘ ‘1 \ Jech 5000 ‘1 1. ' Sraplston A ’ Central 1’ RF“ Stapletun ALrpnn W k' I Him I 1 Al'rpon W k' “I '"5 OBenneI -Em ‘re at "15 lBennEt 'Denver ' 6/ 9' ODenver ' _—'-‘——\\ Englevzood '1 Englmfioud Airport ,,-E\\ \\ Evergreen Airport ’,_-\\ \ (f-“iT’arker \\ /_~__,/' ——-’3/Parker \ B 'I Pine/’J \\\ 5395““ \x ”/1“\ 3 -| Pine""I \\ Se. 1a a1 0y i \ x“: \ / \ a. 8v 7 \ \“1 / \ ‘\ ' ,7 ,—\ ”1‘ 33$ Elllaheth \\ / ‘\ \\ ' ,3 ,3 l,\\ $3: 5112:0201 \ , \ l \ 7 x 10000 10000 \ / I \‘~»_\ Kiuowa \ 10000 10000 ‘\ 1/ /' ’1’ \“m \ Kin‘wa \ / x \ x 0000 / \ ‘7 I'chkers \ \ 6000 \ \ . \\ \ I) / '/ o \\ Elb \ \ Larkspur \ \\ I, 7/ / ‘\ Larkspur “-‘3‘ E" \ 1 \V‘ ‘1 l/ \ \\ 7500 " 7500 7000 I | O 50 MILES l—L—L_L__g__1 EXPLANATION ————0000———— Datum is mean sea level RADAR ECHO, VIDEO-INTEGRATOR PROCESSOR LEVEL 1 RADAR ECHO. VIDEO-INTEGRATOR PROCESSOR LEVEL 3 TOPOGRAPHIC CONTOUR ——Contour imerval, in feet, is variable. FIGURE 43.—Continued. 53 54 FLOOD, JULY 31—AUGUST 1, 1976. BIG THOMPSON RIVER, COLORADO 105” 105" l / ' _______ , [T — / __________ / ,16000 2040 MDT 7500 ,//snuu 2050 MDT / / ,/ 5090 5090 \ «, \ o /"v """""" ,r"v_"“"’” \657/ I) FLICnlllns .ICnlllns °I . ' \ OWindsur ‘ nWindsor «57/ ralge l 3 §/ l PLoveland ‘3'99'9‘1 l'aneland oereelev $/ ’ on \ \ / Grand / Barthou ’ Benhoud ak ' \\ r' z I l ’ \ //—\\ K " lf\ \ \-/ \E'_‘ \J/ “"\\ IBrighwn ‘ anghmn \~\\ _ Jefiw 5000 . Staplemn Staplelon Airport Airpnrl Watkins .Bmfl olGuIdgn ' Watkins oBEnnt ODenver ' ODenver ' e \ l K4” Englw7und Englefiuud \ Evergreen Ai'part ,__\ Airport ,_~\ / \ / \ ,’-—‘3Parker \ \ \ / iParker \ \ \ , l \ / \\ Bailey Pine’E" :\\Sedlalla C \ // r\ Bailey Pine/‘1 :\ Sad/aha \ \ . .1 \ ~— aslle L / \ ‘\ 0/ \“v Castle - \ \\ 1’1/7‘ (A\\\ 0Rnck Elgzabsth' \\ // \\ \ ' 1’] /,~“ /,\\\ oRuck Ellzabeth. \\ \ ‘—’ ,/ / \‘~—~ Kiowa \ moon quuu \ ‘—’ l’ \ ~~—\ mea \ 77,7 l’lzeckers \ \\ 6000 / \ ‘7 IDeckers \ 6000 , \ \\ x / \ ‘\\ \\\ I/ ,’ // \ LZIkspur E“~:\Elbm \\ I] / ll \\ Larkspur 7 :Elherl \V: l ,1, \ \\ \J l // \ \\ V 7500 7000 " 7500 7000 l | 106° 105" l / ' --._-_«' l / ' ........ /’/suun’ 2100 MDT / ,Asuon’ 2110 MDT / / \\ l 50er 5090 A \ {1/1, — ,x \\ . (”i 7777777 \‘ - /"_ ______ (J Ft.|Co|llns ,1 Ft.‘Colllns ' ‘ 'Windsur ‘ IWindsur Doralfe l e l ( {Loveland 'G’EE'EV gLoveland '6'99'" I \ n l ,’ Banhoyd‘ / Benhoyd \ \ " l x [I Table ,1 .Tab'e / Mt. \ fl //—\ M“ \_ , l/‘\\ Boulder \J ‘**-\ Boulder \J ‘*--\ IBrigmou \\\\ _ 'Brighlnn »\\‘ _ JENCU 5000 \ lefco 5000 ‘. ' \ \ u ' “FEEL! 5"P'9‘0“ ,/’ Central r‘ “£1:st Staplotnn \ Alrport _ / . City ,’ \ Alrpon _ -\Gn|den ° Wagkms oBennzl l -\Gu|den ’ Wagklns chum ODenver "\ \ ODenver \ r 12;; Englsfiond I\\ E 9/1: Englefinnd \\ AIVDOH /-_\ \‘ V0199 Airport f- \ \ / \ ,a» Parker ' ,»-—’3Parker \\ \ /, \ / \ \\ Seglalla Sig/Illa \\_‘ V \ \, ‘ I, \ ,'\\\\ 3:3: Euzanexn \\ l/ \ $ng Hymn. \\ mono mqun \ , / \\ ~--\ mam \ moon \ Km'wa \ / \\ ‘7 l/lzeckzrs \ \ 6qu / \ \ soon , / \\ \ \\ \ \ l/ ,' /’ \\ Larkspur \\‘ .5312" \\\ Larkspur ‘ \J l/ \ \ \ V 7500 7000 7500 I I u 50 MILES L—L__l__l_._J—l EXPLANATION RADAR ECHO, VIDEO~|NTEGRATOR PROCESSOR LEVEL 1 RADAR ECHO, VIDEO Central /' “£1:st Staplamn ’/ 'm '/ ‘6 Id Allpm Watk'ns / ”my ‘1, ‘5 Id A?“ w lk' / IEm ire l o\ 0 en | IBennm I ol 0 en a In: oBgnnel C ’ p \\ \ 'Denver ' \\ \ ODenver ' —"_\‘—\ \ \ \' I "‘ \\ En lefioad \‘ / "‘ \\ En Ielfiood ‘\ Evergreen: ‘\ Eirpurl _ ‘l Evergleenl \ "PD" _ \ \ l /' ‘~\ \ \\ K ,/ ‘~\ ’_“_,/’ \ \ -—’;Parker \ 4 /_‘__,/’ \\ \\ ,~--'3Parker ,1 ‘ \ , . \ V / r\ _ . ,__1 \ Sedalla \ \ _ , ,-_, \ Seflalla \ / l \ Barley Plrle mg: \ \ Barley que \3/ \ 1/ \ \\ ' ,7 /~\ (Ah 333$: Elllaheth \\ 1/ \ \\ ' ,‘x A \ .fig'f Elizabeth \\ , \ l I \ worm loqoo \\ v / [I \‘x‘ \ Kio'wa \ moan luquu \\ ‘ l ‘ \ Kic7wa \ / \ ‘7 xlDeckers ‘ \ 6000 / ‘\ \ 6000 \ I I '1’ \ - \‘\ .Elbgn \ 0 \‘\ IElbell \\ ll ,‘ / \\ Larkspur “~~\\ \\ \\ Larkspur ‘*~~\ \\\J l [1/ \\ \ \\ \ \\ V 7500 7000 7500 7000 l l 0 50 MILES \__l~_..J—l__.L.__.J EXPLANATION RADAR ECHO, VIDEO-INTEGRATOR PROCESSOR LEVEL 1 RADAR ECHO, VIDEO-INTEGRATOR PROCESSOR LEVEL 3 TOPOGRAPHIC CONTOUR. —Contour interval, in feet, is variable. Datum is mean sea level FIGURE 43.—Continued. 55 56 FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO storms were generally over the tributaries of the North Fork Big Thompson River. Heavy rainfall reportedly continued in the Glen Haven area until about 2200 MDT with isolated bursts of rainfall near Estes Park and Glen Haven continuing into the early morning hours. Between 2200 and 2300 MDT, maximum rain- fall intensities moved north-northeastward into the foothills west of Fort Collins. During that same general time period (2000—2300 MDT), heavy rainfall occurred over the North Fork Cache la Poudre River basin between Livermore and Virginia Dale. Local residents reported the heaviest rainfall between 2100 and 2200 MDT. As shown in figure 42, moderate rainfall occurred along the southern edge of the Cache la Poudre River basin be- tween 1800 and 2200 MDT, becoming much more in- tense thereafter. Heavy rainfall continued in this area until about 0100 MDT on August 1, with light showers persisting throughout the night. Rain showers con- tinued during August 1 and 2 over most of the general storm area, with locally heavy amounts falling at times, especially near Fort Collins. As mentioned earlier, data defining the rainfall rates in the maximum rainfall zone west of Drake were not obtained. In order to provide some estimate of the time distribution of rainfall that produced the severe flash flooding along the Big Thompson River, Limon radar reflectivity data were used to develop cumulative rain- fall diagrams for Glen Comfort and Glen Haven (fig. 44). Several assumptions were required in order to make the calculations. A total of 12 inches of rain fell at Glen Comfort dur- ing the 3-day storm period, but it is not certain how much occurred during the period of the flash flooding. On the basis of meager information from local residents, it was assumed for calculation purposes that 10 inches of rain fell between 1830 MDT and midnight on July 31. Using the comparison between Grover and Limon radar reflectivity data, it was also assumed that reflectivities within the central half of a Video In- tegrator Processor level-3 area were 2 dBZ higher than the level-3 threshold value. Two generalized relationships were used along with the assumptions and the Limon radar reflectivity data to derive the cumulative rainfall totals. The two rela- tionships are z=200p1‘6 and z= 55pm ’ where: z is the reflectivity factor, in millimeters6 (mm5) per cubic meter, and p is the precipitation rate, in millimeters per hour. The National Weather Service has recently found that for WSR—57 radars, such as the one at Limon, the latter relationship provides more accurate rainfall rates for convective storms. Resulting rainfall amounts from 1830 MDT to mid- night on July 31 at Glen Comfort were 0.98 inch for z=200p1'6 and 2.22 inches for z=55p1'6. Assuming the reflectivity data were low by a constant factor, ad- justments of 16.1 dBZ and 10.6 dBZ, respectively, yield the assumed total of 10 inches of rain. These ad- justments agree closely with the differences indicated by the comparison of the Grover and Limon reflec- tivities. Adjusted Video Integrator Processor levels 2, 3, and 3+ (level-3 threshold +2 dBZ) represent rainfall rates of 1.21, 5.91, and 7.89 inches per hour, respective- ly. These modified rainfall rates were used to construct the accumulated rainfall diagrams for Glen Comfort and Glen Haven shown in figure 44. The diagram for Glen Comfort shows that about 7.5 inches of rain fell between 1930 and 2040 MDT, with rainfall rates being much lower before and after this period. The Glen Haven diagram shows that heavy rainfall began somewhat later in that area, continued for a longer period of time, but fell at slightly lower rates than in the vicinity of Glen Comfort. HYDROLOGIC ANALYSIS OF THE FLOOD The severe thunderstorms of July 31 produced flooding in Larimer County along a band several miles wide extending from just southeast of Estes Park northward to the Wyoming border. The Big Thompson River basin between Estes Park and Drake was especially hard hit by the storm which resulted in devastating flooding along the Big Thompson River from Estes Park to Loveland and along the North Fork Big Thompson River from the Glen Haven area to its mouth at Drake. The flood in the Cache la Poudre River basin originated mainly in sparsely populated areas along the North Fork Cache la Poudre River be- tween Livermore and Virginia Dale; consequently, flood damage was much less extensive and, fortunate- ly, no deaths occurred. FLOOD DATA STATION DESCRIPTIONS AND STREAMFLOW DATA Flood data obtained at 10 gaging stations and 27 miscellaneous sites in the affected area are tabulated in downstream order on pages 72—81. Station descrip- tions give the location of each site, size of the drainage area upstream from the site, the method of discharge determination, and peak discharge or peak stage dur- METEOROLOGY, HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS 57 11IlllllllIITITTIIIIIFIIIIIIIIIII 9 _. GIen Comfort ESTIMATED CUMULATIVE RAINFALL, IN INCHES O'l 2100 Glen Haven IILIIIIIIIIIIIII 2200 2300 2400 0030 July 31 I August 1 TIME, IN HOURS (MDT) FIGURE 44.—Estimated cumulative rainfall at Glen Comfort and Glen Haven, 0010., July 31—August 1, 1976. ing July—August 1976. Where available, information also is given on gage datum, nature of gage-height record obtained during the flood period, and maximum stage and discharge known prior to this flood. PEAK STAGES AND DISCHARGES Peak stages and discharges for the 10 gaging sta- tions and 27 miscellaneous sites in the flood area are listed in downstream order in table 3 and site locations are shown in figure 45. The drainage areas listed for sites on the Big Thompson River downstream from Lake Estes include both total drainage area and the in- tervening area between Lake Estes and the respective site. As subsequently explained, the gates at Lake Estes were closed at the beginning of the flood; thus, the upstream part of the basin did not contribute to the flood. Drainage area for several other sites are foot- noted in table 3 to indicate that only a small, undeter- mined part of the basin contributed to the flood. Also listed for some sites in table 3 are the previously recorded maximum stages and discharges. VELOCITIES AND DEPTHS Probably the most destructive element during the flood period was the extremely high velocities of the floodwater. These high velocities greatly increased stream competence resulting in severe channel erosion and transport of debris and large streambed material. Average velocities of 20-25 feet per second were com- mon on the steep tributaries near the center of the rainstorm. In general, average cross-sectional depths were small with most depths being less than 6 feet ex- cept on the main streams where depths as much as 10 feet occurred. Average velocities and depths for 29 sites where slope-area measurements were made are listed in table 4. 58 FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO 1 05° I I ! LARAMIE ‘ @ 0 V H EYEN N E 13 41° — — — - — 2v 6",ng Cr «6‘ m e°‘”“ Re Feather a w,.%,, \ 67460177 Creek Rustic ‘i - “I; )vJ-wxufvl‘r ‘\ A R v~ M “ R | on\ «fi . CarN Laporte . R's : a, .. o 35 ' Sold/e! WELD ab“; Canyon 36 6 6,” °% For mums; ( A 7 «2+6 Horsetoozhfi '3 fit, ° I § ’5 A9 o as ‘ I; Eaton h 10 c 4/0 25 k 25;. W 6% 18 Masonwll v x &, 14 13 19» 260 w 2 w 17 “4 21 22 Loveland w Q9055 INSET— 16 20 D a e Power Plant24 I la Poudf" Rive 37 4 . 1 1 shame I r G Co 7 8 Midlvzay 23 Gil Gr 5 edaf Mouth of Big OV I U oveland lghIS CW9 Thompson Canyon n I 94“ 1 2 0 g us Heights 3 927 The Na rows n L k I. _ _ - 23 L er a e Res I “o“ 4 Little 7170me W 55‘ 3‘ g‘v’d l a) (no . m — — - — — - — - o —- — v , BOULDER 1 Base modified from use Geological Survey 0 10 12500000, State base map, 1969 20 MILES | I l EXPLANATION Station and site numbers indentified in tables 3 and 4 A3 STREAM—GAGING STATION AND NUMBER A7 MISCELLANEOUS MEASUREMENT SITE AND NUMBER FIGURE 45.—Stream-gaging stations and miscellaneous measurement sites in flood area in Larimer and Weld Counties. METEOROLOGY, HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS 59 1 05°30’ 1 4,. .3 40°30 — ”6, :g ~ ‘8 W o 5 1°84 figs 0 v“ . A °‘« 3* <9 19 0 F0 13 Glen . A 6 was t x 1 oHaven Bug Q0,b S}. 09!} A A17 4‘00 ~¢ 20 o \0 .- . . -—A\. . . 9 I~%4 1 5A 0° . / \ . . . g- A 16 r 21 creek Em Drake ' A22 Cow o" K 12 (0,, A ._,9 as \ Gale/7 ' I “00/ Waltgma 35“ .-' \oa % A1 1 O / '5 River A4 : / A . a) 0 ’9 10 \ .' E 9 . o O 6 '. -,. 7 8 3' oé‘ gr A A K "“ \ E 8:19 ' 9 1. \ Q Loveland A Glen Oo\ -- .w Heights Comfort 6,, v Esta: 5% - x -. Park 0 1 Q :2 1 s’ .- 2 - ~17 g. Q ‘i U 5 MILES | I 1 I | 1 FIGURE 45.—Continued. 60 TABLE 3.—Flood stages and discharges in Larimer and Weld Counties for the flood ofJuly 3hAugust 1, 1976, and during previous maximum floods FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER. COLORADO Maximum floods previously known Maximum July 314August 1, 1976 Prior to J uly- Site Station Stream and place D - A 1 76 Ga e . Recurence . G . Rec No. No. of determination ramage ugust 9 heig t Discharge interval Dlscharge Hour 11 9 Discharge _ urence . h t a 1 (Br-:18) Perlod Year 1ft) (ft ls) (yrs! day (MDT) 96% (ft lsb 111132133}: 1 06733000 Big Thompson River a b 00 F altilésteiPark ..... 137 1946-76 1949 3.16 1,660 13 31 2130 3.64 457 <2 2 067345 is ree near Estes Park ....... “16.0 1947-55, 1951 7.32 €1,480 d3.7 31 2150 4.02 182 3 1976 3 06735500 Blg Thompson River b d / nearEstes Park . . . 155 1930-76 1933 4.0 2.800 1.1 31 2230 5.51 (g) ....... 2 (0) 4 ....... Dr Gulch near D séesllznk ....... 2.00 ................................... 31 "2215 3.210 ....... 5 ....... u c at stes Park ....... 6.12 ................................... 31 "2230 ....... 4,460 ....... 6 ....... Big Thompson River h below Estes Park . 164’ ................................... 31 2300 ....... 4,330 75 9 7 ....... Big Thompson River tributary below Loveland Heights . 1.37 ................................... 31 .............. 8,700 ....... 8 ....... Dark Gulch at Glen Comfort ..... 1.00 ................................... 31 .............. 7,210 ....... 9 ....... Noels Draw at Glen Comfort ..... 3.37 ................................... 31 .............. 6,910 ....... 10 ....... Rabbit Gulch near Drake ........... 3.41 ................................... 31 .............. 3.540 ....... 1 1 ....... Long Gulch near B Drallite ..... .R. . . . 1.99 ................................... 31 .............. 5,500 ....... 12 ....... ig T ompson iver above Drake ...... 9(1389 ................................... 31 ”2100 ....... 28.200 ”13.8 4) 13 ....... North Fork Big Thom son River at Glen aven ...... “ 18.5 ................................... 31 .............. 888 ....... 14 ....... Fox Creek at Glen Haven ...... “7.18 ................................... 31 ....... 1,300 ....... 15 ....... West Creek near Glen Haven ...... “23.1 ................................... 31 ....... 2.320 ....... 16 ....... Devils Gulch near Glen Haven ...... .91 ................................... 31 .............. 2,810 ....... 17 ....... North Fork Big Thompson River tributary near Glen Haven ...... 1.38 ................................... 31 .............. 9,670 ....... 18 ....... Black Creek near h Glen Haven ...... 3.17 ................................... 31 2300 ....... 1.990 ....... 19 ....... Miller Fork near a )1 Glen Haven ...... 13.9 ................................... 31 2300 ....... 2,060 ....... 20 ....... North Fork Big Thompson River tributary near Drake ........... 1.26 ................................... 31 .............. 3,240 ....... 21 06736000 Nolrfih Fork 811g. ompson 1ver . at Drake ......... “85.1 ‘1947-76 1965 5.66 1,290 8 31 2140 9.21 8.710 d1.4 22 ....... Big Thompson River a d below Drake ...... 3‘ 276) ................................... 31 .............. 30,100 2.9 121 23 06738000 Big Thompzon fRiver at mout 0 can- yon, near Drake .. a 305 1887-92, 1919 ....... 18,000 26 31 h2140 19.7 31,200 d1.8 150} 1895-1903, 1926-33. 1948-49, 1951-76 24 ....... Big Thompson River lgellogv Green Ridge a e. near .......... Loveland......... :311j 31 27.000 “1.7 (150 25 ....... Redstone Creek B nelfxl'1 Maéonvlille . . "29.1 ................................... 1 .............. 2,640 5 26 06739500 uc orn ree near Masionville . . “131 1933, 1951 13.40 14.000 50 l ....... 8.1 3,400 4 9 8, 1947-55 27 ....... Ligle Thompéon iver near stes Park ............. 2.77 ................................... 31 " 2130 ....... 1.940 ....... 28 06744000 Big Thom son River at mout . near LaSalle .......... 828 1914-15, 1951 (17.80 6,100 77 1 2235 7.82 2.470 11 “16731 192776 29 ....... Dale Creek tributary near Vir 'nia Dale .68 ............................ 31 .............. 727 17 30 ....... Deaclnlan reek near a h Vlrglnla Dale ..... 23.7 ............................ 31 2230 ....... 7,400 100 31 ....... Stonewall Creek a )1 near leermore . . . 31.9 ................................... 31 2200 ....... 3,470 6 32 ....... Lone Pine Creek a h near Livermore . . . 86.3 ................................... 31 2230 ....... 2,590 6 METEOROLOGY, HYDROLOGY. BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS 61 TABLE 3,—Flood stages and discharges in Larimer and Weld Counties for the flood ofJuly 31—14 ugust 1. 1976, and during previous maximum floods —Continued Maximum floods previously known Maximum July 31-August 1. 1976 . . Prior to July- Site Station Stream and place - Ga e . Recurence . . n neeenninin masses ——A“2“ ”1.222189 nervei ”1532239 .1131. .213: ”1129239 39:33:: (mi 2) Period Year 1ft) (yrs) y m) WIS) 33 ....... North Fork Cache la Poudre River at Livermore ........ “539 1904, 1904 ....... 20.000 33 31 .............. 9.460 16 1929-31 34 06752000 Cache 1a Poudre River at mouth of can on. near Fort a k 0 'ns ........... 1.056 1882- 1904 ....... ( l ....... 1 0130 7.86 7.340 16 1976 35 ....... Rist Canyon near h Bellvue .......... 5.27 ................................... l 0030 ....... 2,710 16 36 06752260 Cache la Poudre River at Fort a Collins ........... 1.129 1976 ............................ 1 0430 8.84 5.700 ....... 37 06752500 Cache la Poudre a I River near Greeley 1.877 1903-04 1917 ....... 4.220 ....... 2 0030 5.62 1.600 ....... 2Contributing drainage area for flood of July 3Mugust 1. 1976. unknown. Site and datum then in use. SCaused by dam failure at Lilly Lake. Ratio of peak discharge to that of 100-year flood dischar e. prproximate contributiniarea during flood of July 31—- ugust 1. 1976. Backwater from Dry Gulc . fiGates at Lake Estes closed at 2055 MDT; no outflow during remainder of flood period. . Approximate time based on information from local resident. {Record unpublished 1956-76; availiable from State Engineer's office. iRecorded at site 5 miles upstream. Greater than 21.000 cubic feet per second. Daily discharge. TABLE 4.—Hydrologic data for selected flood-data sites TABLE 4.—Hydrologic data for selected flood-data sites—Continued Unit Unit - - . Drainage - dischar 9 Average Average Ste Station . Draina e . dischar e Average Average i113: Stfigfm Stream and Location area D'fi‘g‘gge ((ft‘ls) velocity depth Nlo. No. Stream and Location areag DlstE/arge {Ift’lsll velocity d9 th 1m?) mi”) (ft/s) ( 0 (mi’) ‘ t 5’ min) (ft/s) 1 :1 4 ........ D Gulch near Estes 23 06738000 Big Thompson River ark. Colo ........ 2.00 3.210 1.600 12 3.3 :gnfizglbgkecani ........ B' ' . . 6 1563;352:153? Colo ............. bsos 31.200 ..... 26 10.6 Colo ............. a 9 4.330 481 8 4.6 . . 7 ........ Big Thompson River 24 ........ Big Thompson River tributary below 16:1?" Green Ridge 1 H . ht , a e. near an? arid ‘ 91g“ .s_ . 1.37 8.700 6.350 26 5_5 Loveland. Colo . . . b311 27.000 ..... 12 6.7 8 ........ Dark Gulch at Glen Comfort, Colo . . . . 1.00 7,210 7.210 23 5.1 25 -------- Redstone Creek near b 9 ....... Noels Draw at Glen .Masonvdle. Colo . 29.1 2.640 ..... 10 4.2 (among Colo . . . . 3.37 6.910 2.050 21 5.7 27 -------- Ugilge’lr‘mrpégges 10 ........ Ra it G ch nea _ Drake. Colo ...... 3.41 3,540 1.040 13 4.7 Park 0010 -------- 2-17 1'940 700 10 1-6 11 Long Gulch near 30 ........ Deadnian Creek near b Drake. Colo ...... 1.99 5.500 2.760 19 5.8 Vflsmw Dalen Colo 23-7 7.400 ----- 10 4.0 12 Big Thompson River 31 ........ Stonewall Creek near b above Drake. Colo. 189 28.200 829 22 3.3 LivesmoreeColo . - ~ 31-9 3.470 12 3.7 a 34 32 ........ [glue Pine Cregklnear 1'86 3 13 ........ N h F kB' wermore. o o . . . . 2,590 ..... 7 3.5 elsl‘iomogon give! at 33 ........ North Forkpache la Glen aven. Colo . " 18.5 888 ..... 8 2.2 Ppudre River at b 14 ........ Fox Creek at Livermore. Colo . . 539 9.460 ..... 9 5.3 Glen Haven. Colo . 177.18 1.300 ..... 9 2.8 34 06752000 03011913 Poudre 15 ........ West Creek near to River at ”‘0“? 0‘ G] H .c I . 23.1 2. 20 7 3.0 can ,on. near 0“ 16 ........ 1335305313333? ° 3 .00 ns, Colo ----- ”1.056 7.340 9 62 Glen Haven. Colo . .91 2,810 3,090 12 2.1 35 """" Rlst Canyon near 17 ....... North Fork Big Bellvue. Colo ..... 5.27 2.710 514 12 3.4 Thompson River 36 06752260 C5133!“- 1a 1:?‘129 tributary near Glen “’2" a or b Blnygn' (i010 ______ 1.38 9.670 7,010 29 53 Collins. Colo ..... 1,129 5.700 ..... 8 6.7 18 """" G‘len liesvethaéolo . 3.17 1,990 628 11 4'5 2 Approximate contributing drainage area during flood of July 31-August 1. 1976. 19 Miller Fork near Glen Contributing drainage area for flood of July 31—August 1. 1976. unknown. Haven. Colo ...... " 13.9 2,060 ..... 12 3.6 20 ........ North Fork Big Thompson River tribut near Drake. olo ...... 1.26 3.240 2.570 18 3.0 21 06736000 North Fork Big Thompson River I: at Drake. Colo . . . . 85.1 8.710 ..... 12 5.2 22 ........ Big Thompson River b below Drake, Colo. 276 30.100 ..... 16 10.3 62 FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO FLOOD MARKS AND PROFILES Soon after the flood, the US. Geological Survey, the US. Army Corps of Engineers, Omaha District, and the US. Bureau of Reclamation referenced high-water marks at numerous locations along the Big Thompson and the North Fork Big Thompson Rivers. The Col- orado Water Conservation Board conducted a study to develop flood profiles from the high-water marks and stream cross-section data at selected locations in the flood area. Flood profiles, cross-section data, and preliminary streamflow data are contained in a report published by the Colorado Water Conservation Board (Grozier and others, 1976). DETAILED DESCRIPTION OF FLOOD AREAS BIG THOMPSON RIVER BASIN: UPSTREAM FROM OLYMPUS DAM After the flood, a field inspection made upstream from Olympus Dam indicated that streams west of Estes Park received no appreciable flood runoff. The gaging station on the Big Thompson River upstream from Lake Estes (Site 1) recorded a peak discharge of 457 cubic feet per second at 2130 MDT as shown in figure 46, but all this floodwater reportedly was stored in Lake Estes and did not contribute to the downstream flooding. Earlier, at 1930 MDT, a much smaller peak had occurred, and an intermediate-sized peak was recorded at 0300 MDT on August 1. Minor flooding also occurred on Fish Creek (Site 2) which enters Lake Estes from the south, just upstream from Olympus Dam. The discharge hydrograph for Fish Creek (fig. 47) shows that the stream rose slightly at 1920 MDT with a lull until 2000 MDT, followed by a sharp rise which peaked at 2150 MDT at a discharge of 182 cubic feet per second. A smaller peak occurred on Fish Creek at 0400 MDT on August 1. The gates at Olympus Dam were closed at 2055 MDT on July 31 at which time the river discharge was about 200 cubic feet per second. This discharge is a small percentage of the peak discharges at downstream sites; thus, the basin upstream from Olympus Dam was considered as a noncontributing area for the purpose of these analyses. BIG THOMPSON RIVER BASIN: OLYMPUS DAM TO LOVELAND HEIGHTS This part of the flooded area consists of approx- imately 9 square miles beginning at Olympus Dam (Site 3) and ending at Site 6 on the Big Thompson River 0.5 mile southwest of Loveland Heights. Dry Gulch, which drains 6.12 square miles northeast of Estes Park, flows southwestward to join the Big 500 I I T 300 — — 200 — _ DISCHARGE, IN CUBIC FEET PER SECOND 100 -— _ 0 J | 2400 1200 2400 JULY 31 l 1200 AUGUST 1 2400 1976 FIGURE 46.—Discharge hydrograph for the Big Thompson River at Estes Park (Site 1). Thompson River just downstream from Olympus Dam. A peak discharge of 3,210 cubic feet per second occurred at Site 4 on Dry Gulch about 2.4 miles upstream from the mouth. According to a local resi- dent, the peak stage occurred at 2215 MDT. Downstream at US. Highway 34, Dry Gulch (Site 5) had a peak discharge of 4,460 cubic feet per second. The peak stage occurred about 2230 MDT at Site 5 ac- cording to the gaging-station record on the Big Thompson River downstream from Olympus Dam (fig. 48). As shown in figure 49, the floodwater from Dry Gulch impinged on the base of Olympus Dam just downstream from US. Highway 34, eroding part of the base material. Fortunately, the erosion did not create a dangerous situation. The largest rainfall amounts in Dry Gulch basin oc- curred upstream from Site 4 resulting in a unit discharge of 1,600 cubic feet per second per square mile, from the 2.0-square-mile basin. Much of the Dry Gulch basin between the two sites lies to the west of the area of heaviest rainfall; thus, the unit discharge METEOROLOGY, HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS 63 200 | I I 160 — _. Q 2 O U m (D E 9- 120 — — ,— Lu Lu u. 9 m D U E _ 80 — _. u.I (D I < I U 1’ D 40 — — o I J l I 2400 1200 2400 1200 2400 JULY 31 AUGUST 1 1976 FIGURE 47.—Discharge hydrograph for Fish Creek near Estes Park (Site 2). was much less for the downstream site, being 729 cubic feet per second per square mile. The peak discharge on the Big Thompson River upstream from Loveland Heights (Site 6) was 4,330 cubic feet per second. Residents of the area reported that the highest river stage occurred about 2300 MDT which indicates that this rise was derived primarily from Dry Gulch discharge. They also mentioned a smaller rise after midnight, but none prior to the 2300-MDT peak. Considering the above information, the basin upstream from Site 6 probably contributed very little runoff to the initial, and most destructive, flood crest. BIG THOMPSON RIVER BASIN: LOVELAND HEIGHTS T0 DRAKE The tributaries of the Big Thompson River in the upstream part of this reach received the brunt of the July 31 storm. From Loveland Heights to a point about 1 mile west of Waltonia, all tributaries produced extremely high runoff rates and consequent high 5 I I I '— LU I.” M. Z I; I _ g 3}— LU I 5 < (D 2 _ 1 — _ 0 l | I 2400 600 1200 1800 2400 JULY 31, 1976 FIGURE 48.—Stage graph for the Big Thompson River near Estes Park (Site 3). velocities with accompanying streambed and bank ero- sion. In contrast, tributaries east of Waltonia ex- hibited little evidence of flooding. The most severe flooding occurred on north bank tributaries between Loveland Heights and Glen Com- fort. An unnamed tributary (Site 7) near Loveland Heights had a peak discharge of 8,700 cubic feet per second while Dark Gulch at Glen Comfort (Site 8) had a peak discharge of 7,210 cubic feet per second. A peak discharge of 6,910 cubic feet per second occurred on Noels Draw (Site 9) which enters the Big Thompson River from the south about 0.25 mile downstream from the mouth of Dark Gulch. Both the unnamed tributary (Site 7) and Dark Gulch (Site 8) exceeded previously recorded maximum unit- discharge rates for basins of less than 4 square miles in Colorado. The unnamed tributary which drains 1.37 square miles had a unit discharge of 6,350 cubic feet per second per square mile. The unit discharge for Dark Gulch which drains 1.00 square mile was 7,210 cubic feet per second per square mile. Although N oels Draw (Site 9) lies just south of Dark Gulch, the unit 64 FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO FIGURE 49.—Erosion along base of Olympus Dam caused by Dry Gulch floodwater. discharge was much less, being 2.050 cubic feet per second per square mile from the 3.37-square-mile basin. The reason for this large difference in unit discharges is uncertain, but it probably is related to velocity and direction of storm movement past the opposite-facing valley slopes. The peak discharge for Rabbit Gulch (Site 10), a south bank tributary which enters the river 2.5 miles downstream from Noels Draw, was 3,540 cubic feet per second. Both an aerial reconnaissance and aerial- photograph interpretation indicated little flood runoff in Rabbit Gulch upstream from a point about 1.5 miles above the mouth. Farther east, Long Gulch (Site 11), which enters the river from the north, had a peak discharge of 5,500 cubic feet per second. Although Long Gulch heads near the same point as Dark Gulch, probably only the downstream part of the basin re- ceived extremely heavy rainfall. The only stream east of Long Gulch that discharged significant floodwater was True Gulch which enters the Big Thompson River 0.5 mile west of Waltonia. Because smaller streams near Waltonia were relatively unaffected by the storm, it appears that the flood on True Gulch originated near the upper end of its basin and that little contribution came from the downstream part of the basin. The Big Thompson River at Site 12, about 0.5 mile upstream from Drake, had a peak discharge of 28,200 cubic feet per second at about 2100 MDT on July 31. As mentioned previously, the part of the basin upstream from Olympus Dam did not significantly contribute to the flood-peak discharge; therefore, the contributing drainage area for Site 12 is 34 square miles. On this basis, the unit discharge for Site 12 is 829 cubic feet per second per square mile. NORTH FORK BIG THOMPSON RIVER BASIN: GLEN HAVEN VICINITY TO DRAKE Extremely heavy rainfall over an area of approx- imately 20 square miles centered slightly east of Glen Haven produced severe flooding on the North Fork Big Thompson River and several of its tributaries. Heavy rainfall reportedly began about 1930 MDT in Glen Haven, but the first account of extreme flooding came from Fox Creek, which reached a peak stage at about 2100 MDT. Another burst of rainfall at approximately METEOROLOGY, HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS 65 2300 MDT produced rises on several streams with resultant crests almost as high as the earlier ones. The major damage occurred in Glen Haven from West Creek which enters the town from the southwest. Downstream from Glen Haven, damage was limited mainly to the highway, which generally parallels the river to Drake. The peak discharge of 888 cubic feet per second, which occurred on North Fork Big Thompson River about 0.1 mile upstream from Fox Creek (Site 13), in- dicates the small contribution to downstream floodflows from that part of the basin west of Glen Haven. Similar results were obtained for Fox Creek where a peak discharge of 1,300 cubic feet per second occurred at Site 14, 0.2 mile above the mouth. Peak discharges on West Creek (Site 15) and Devils Gulch (Site 16) were 2,320 cubic feet per second and 2,810 cubic feet per second, respectively. Again, only a small part of the West Creek basin was hit by the heavy rain- fall. Conversely, the entire basin of Devils Gulch received extremely heavy rainfall resulting in the high unit runoff of 3,090 cubic feet per second per square mile from the 0.91-square-mile basin. Downstream from Glen Haven, the unnamed tributary which enters the river from the south at Glen Haven picnic ground (Site 17) had a peak discharge of 9,670 cubic feet per second from a drainage area of 1.38 square miles. This peak-runoff rate of 7,010 cubic feet per second per square mile is the second highest unit- runoff rate for the flood, being exceeded only by Dark Gulch (Site 8) which heads due south of this stream. Peak discharges for Black Creek (Site 18) and Miller Fork (Site 19) were 1,990 cubic feet per second and 2,060 cubic feet per second, respectively. The peak stages occurred on these two streams about 2300 MDT; both streams were reported as being dry during the 2100-MDT flood period. A peak discharge of 3,240 cubic feet per second occurred on the unnamed tributary (Site 20) which enters North Fork from the south about 3.2 miles west of Drake. The unit discharge of 2,570 cubic feet per second per square mile is less than that of other streams which head in the same general area, but much of this basin lies near the eastern edge of the storm area. Flood runoff in streams east of this point rapidly decreased with no evidence of flood runoff on North Fork tributaries at Drake. At Drake, the North Fork Big Thompson River (Site 21) had a peak discharge of 8,710 cubic feet per second. This peak discharge greatly exceeds the previous max- imum discharge of 1,290 cubic feet per second which occurred on June 16, 1965. The hydrograph from the gaging station (operated by the Colorado Division of Water Resources) on the North Fork at Drake (fig. 50) shows that one peak occurred at 2110 MDT, possibly caused by backwater from the Big Thompson River. The stream then receded until 2135 MDT and rose again to a peak stage of 9.21 feet at 2140 MDT. The second peak which occurred in the vicinity of Glen Haven about 2300 MDT was not recorded at this site because of the plugged gage intakes. BIG THOMPSON RIVER BASIN: DRAKE TO MOUTH OF CANYON The peak discharge of the Big Thompson River about 0.4 mile east of Drake (Site 22) was 30,100 cubic feet per second. The flood crest at this location occur- red a few minutes after 2100 MDT or approximately 30 minutes prior to the flood crest on the North Fork at Drake. At 2100 MDT, the approximate time of the Big Thompson River crest stage at Drake,the discharge of the North Fork was about 4,500 cubic feet per second. By adding this discharge to the peak discharge of the Big Thompson River at Site 12, a discharge of 32,700 cubic feet per second is obtained, which is only about 9 percent greater than the measured peak discharge of 30,100 cubic feet per second. Fortunately, the flood 9000 , , I 8000 - _ 7000 6000 5000 — _ 4000— _ 3000 DISCHARGE, IN CUBIC FEET PER SECOND 2000 l l 1000 — [H' 1 800 2400 I l 0 2400 600 1200 JULY 31, 1976 FIGURE 50.—Discharge hydrograph for the North Fork Big Thompson River at Drake (Site 21) until 2300 MDT on July 31,1976. 66 FLOOD, JULY 31—AUGUST 1, 1976. BIG THOMPSON RIVER. COLORADO crests on the main stem and the North Fork did not oc- cur simultaneously at Drake; otherwise, the downstream peak discharge would have been larger and damages even more severe. The flood crest moved through the 7.7-mile reach between Drake and the canyon mouth in about 30 minutes for an average travel rate of 15 miles per hour, or about 23 feet per second. The peak discharge occur- red at the gaging station at the mouth of the Big Thompson Canyon (Site 23) about 2140 MDT on July 31 at 31,200 cubic feet per second, as shown in figure 51. Because the gage was destroyed during the sharp rise, river stages were based on observer readings and high-water marks left by the flood, the times were based on information from observers of the flood. The US. Bureau of Reclamation’s calculation of peak discharge at the canyon mouth yielded 30,000 cubic feet per second, a percentage difference of less than 4 percent from that determined by the US. Geological Survey. The peak discharge of 31,200 cubic feet per second is almost four times that of the previously known max- 35,000 1 , I (D Observer readings Peak discharge 31,200 cubic _ feet per second at 2140 1 30,000 I 25,000 20,000 I 15,000 Estimated 10,000 — — DISCHARGE, IN CUBIC FEET PER SECOND 5000 — _ Recorded l l I 600 1200 1800 JULY 31, 1976 0 2400 2400 FIGURE 51.—Discharge hydrograph for rising stage at the Big Thompson River at mouth of canyon, near Drake (Site 23). imum discharge of 8,000 cubic feet per second which occurred on July 31, 1919. The gaging station at the mouth of the canyon was not in operation during 1919, but the flood was recorded at a gaging station about 5 miles upstream. The 1976 peak discharge is more than four times the previous recorded maximum discharge of 7,600 cubic feet per second which occurred on July 19, 1945, at the mouth of canyon gage. The peak stage of 19.7 feet on July 31, 1976, exceeded the previous recorded peak stage by more than 10 feet. BIG THOMPSON RIVER BASIN: MOUTH OF CANYON TO SOUTH PLATTE RIVER As it leaves the canyon, the Big Thompson River valley widens rapidly. The flood crest attenuated quickly because of valley storage and overflow into numerous reservoirs near the river. The peak discharge of 27,000 cubic feet per second for the Big Thompson River just downstream from Green Ridge Glade (Site 24) indicates the effects of peak-discharge attenuation (nearly 14 percent) although Site 24 is only about 2 miles downstream from the canyon mouth. Buckhorn Creek experienced only minor flooding upstream from Redstone Creek. At Site 25 on Redstone Creek near Masonville, a peak discharge of 2,640 cubic feet per second occurred some time after midnight as the storm system moved northeastward out of the Estes Park—Glen Haven area. Downstream from Redstone Creek, a peak discharge of about 3,400 cubic feet per second occurred at the discontinued gag- ing station on Buckhorn Creek (Site 26) south of Masonville. This peak discharge occurred between 0300—0330 MDT based on a stage record furnished by the Colorado Division of Water Resources for Buckhorn Creek (fig. 52) at a site 1.7 miles downstream. As the flood crest on the Big Thompson River moved eastward into the wider and flatter valley, the result- ant reduced velocity caused massive amounts of debris and sediment to be deposited. In Loveland, flooding was limited to low-lying areas along the river and overtopping of several streets. US. Highway 287 south of Loveland was overtopped for a distance of about 0.6 mile at the time of the flood crest. The Little Thompson River, which enters the Big Thompson River about 5 miles upstream from the mouth, experienced severe flooding in the headwaters southeast of Estes Park. The affected area which lies on the south-facing slope opposite Noels Draw was subjected to extremely high runoff rates near the basin divide, as evidenced by numerous small areas of severe sheet erosion. A peak discharge of 1,940 cubic feet per second occurred on the Little Thompson River at Site 27 southeast of Estes Park at about 2130 MDT. The METEOROLOGY. HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS 67 5 I I I I I 5— _ 4— _ E LU U. E E 9 3‘ ' I.IJ I Lu 0 < o 2 — _ 1 )— _ 0 l | | J I 2400 1200 2400 1200 2400 1200 2400 JULY 31 AUGUST 1 AUGUST 2 1976 FIGURE 52.—Stage graph for Buckhorn Creek near Masonville (downstream site). unit discharge of 700 cubic feet per second per square mile from 2.77 square miles of the basin does not in- dicate a high runoff rate; thus, apparently only the ex- treme northwest part of the basin received heavy rain- fall. Farther east, at US. Highway 287 south of Berthoud, the Little Thompson River did not overtop the main channel banks. The peak discharge of the Big Thompson River at its mouth near LaSalle (Site 28) was 2,470 cubic feet per second at 2235 MDT on August 1 (fig. 53). This small discharge is further evidence of the flood-reduction ef- fects of valley and reservoir storage. The flood crest slowed considerably and traveled through the 35-mile reach between the canyon mouth and the mouth of the river in about 25 hours, an average rate of 1.4 miles per hour. CACHE LA POUDRE RIVER BASIN Flood runoff originated from an area about 10 miles wide extending from north to south across the Cache la Poudre River basin. Generally, US. Highway 287 traverses the area from northwest to southeast (fig. 2). The two heaviest areas of flood runoff appeared to be on tributaries of the North Fork Cache la Poudre River north of Livermore and on streams in the vicinity of Bellvue, northwest of Fort Collins. The rainfall that produced the flooding on the North Fork tributaries near Virginia Dale began about 2000 MDT and was reported as being most intense between 2100 and 2200 MDT. Between 2200 on July 31 and 0100 MDT on August 1, the part of the storm system that caused the Big Thompson River flooding moved over the Bellvue ' area and produced flooding in Rist Canyon, on several Cache la Poudre tributaries near Bellvue, and on the downstream tributaries of the North Fork Cache la Poudre River. The western limit of the flood on the Cache la Poudre River was in the vicinity of Poudre Park, while on the North Fork Cache la Poudre River no significant flooding occurred upstream from the mouth of Dale Creek just southwest of Virginia Dale. There was minor flooding along the headwaters of Box- elder Creek east of Virginia Dale, but data were not ob- tained for this basin. According to information from local residents, tributaries of the North Fork Cache la Poudre River basin near Virginia Dale crested between 2200 and 2300 MDT on July 31. Deadman Creek at US. Highway 287 (Site 30) crested about 2230 MDT at a peak discharge of 7,400 cubic feet per second. The floodwater at this location eroded the highway fill behind one bridge abutment and temporarily halted traffic, but the bridge was not overtopped or struc- turally damaged. A peak discharge of 727 cubic feet per second, or 1,070 cubic feet per second per square mile, was determined for Dale Creek tributary (Site 29) about 1 mile northwest of Virginia Dale. Stonewall Creek (Site 31) near the mouth experienced a peak discharge of 3,470 cubic feet per second about 2200 MDT. Much of Stonewall Creek basin east of US. Highway 287 did not contribute significant flood runoff. Lone Pine Creek west of Livermore (Site 32) crested about 2230 MDT at a peak discharge of 2,590 cubic feet per second. On the basis of field inspection and rainfall data, only the eastern edge of the Lone Pine Creek basin contributed to flood runoff. The flood crest on North Fork Cache la Poudre River at Red Feather Lakes road (Site 33) occurred about 0100 MDT on August 1 at a peak discharge of 9,460 cubic feet per second. According to a local resident, the river at this location was higher than at any time since May 20, 1904, when a peak discharge of 20,000 cubic feet per second occurred. The North Fork reportedly crested at its mouth about 0200 MDT on August 1. Flooding on the main stem of the Cache la Poudre River downstream from the mouth of the North Fork resulted from a combination of flood runoff from the 68 FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER. COLORADO 2500 ‘ I I 2000 1500 1000 DISCHARGE, IN CUBIC FEET PER SECOND 500 0 1 I 1 l l 2400 1200 2400 1200 JULY 31 AUGUST 1 l 1200 AUGUST 3 I 2400 1200 2400 2400 AUGUST 2 1976 FIGURE 53.—Discharge hydrograph for the Big Thompson River at mouth, near LaSalle (Site 28). two periods of intense rainfall. The discharge hydrograph (fig. 54) for the gaging station on the Cache la Poudre River at the canyon mouth near Fort Collins (Site 34) shows that the river rose only slightly until about midnight on July 31. After midnight, the river rose sharply to a peak discharge of 7,340 cubic feet per second at 0135 MDT on August 1, about 30 minutes prior to the flood peak on North Fork at its mouth about 4 miles upstream. Because the hydrograph shows only a single peak, it appears that the peak rates of flood runoff from the two intense bursts of rainfall arrived almost simultaneously at the gaging station. Several south bank tributaries of the Cache 1a Poudre River between Laporte and Poudre Park yield- ed high flood-runoff rates. Rist Canyon, which enters the Cache la Poudre River just upstream from Bellvue, reached a peak discharge of 2,710 cubic feet per second about 0015 MDT on August 1 at Site 35. The unit discharge for this 5.27-square—mile basin was 514 cubic feet per second per square mile. The period of most in- tense rainfall in this basin reportedly occurred between 2315 and 2345 MDT on July 31. A peak discharge of 5,700 cubic feet per second oc- curred at the gaging station on the Cache 1a Poudre River at Fort Collins (Site 36). The discharge hydrograph for the station (fig. 55) shows that the river started to rise sharply at 0200 MDT and crested at 0430 MDT on August 1. Farther downstream, the Cache 1a Poudre River at the gaging station near Greeley (Site 37) reached a peak discharge of 1,600 cubic feet per second at 0030 MDT on August 2 (fig. 56). As in the Big Thompson River basin, the sharp reduction in peak discharge between the upstream sta- tion and the mouth of the river indicates the at- tenuating effects of valley and off-channel storage. METEOROLOGY. HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS 69 3000 I I I I I 7000 - -I 6000 I l 5000 — _ 4000 - - 3000 I I DISCHARGE, IN CUBIC FEET PER SECOND 2000 1000 — __J 0 I I | I L . 2400 1200 2400 1200 2400 1200 2400 JULY 31 AUGUST 1 AUGUST 2 1976 FIGURE 54.—Discharge hydrograph for the Cache la Poudre River at mouth of canyon, near Fort Collins (Site 34). FLOOD FREQUENCY In some parts of the flooded area, the peak discharges of the July 31—August 1, 1976 flood are ex- tremely rare, but, in other parts, they have been ex- ceeded several times. Along the Big Thompson River from Estes Park to the canyon mouth, this flood far ex- ceeded the previous maximum flood recorded during almost a century of documented observation. Con- versely, the flood on the Cache la Poudre River has been exceeded several times during that same period of time. To provide an estimate of the probability of oc- currence of the flood, a frequency analysis was made for gaging-station records, and regional regression equations (McCain and Jarrett, 1976) were used for ungaged sites, where applicable. The flood-frequency results are listed in table 3 for sites where estimates could be made. The 1976 flood discharge at the canyon mouth (Site 23) of the Big Thompson River was 1.8 times the 100-year flood for that site. Upstream in the Big sooo , , , 5000 — - 4000 — _. 3000 L _ I I 2000 DISCHARGE, IN CUBIC FEET PER SECOND 1 000 _ x J . l I 2400 1200 2400 1200 2400 JULY 31 AUGUST 1 1976 FIGURE 55.—Discharge hydrograph for the Cache la Poudre River at Fort Collins (Site 36). Thompson River basin near the flood source, the flood was even more rare than at the canyon month. For some sites in this area, the ratio of the 1976 peak discharge to that of the 100-year flood discharge was computed as listed in table 3. At the Big Thompson River above Drake (Site 12) the 1976 peak discharge was 3.8 times the estimated 100-year flood discharge for the site. For several small basins in the Estes Park—Glen Haven vicinity, no estimates of the 100-year floods could be made because existing methods are not applicable, but on the basis of field data and information from local residents, the floods in these basins probably were much greater than the 100-year flood. In the Cache la Poudre River basin, the floods on Deadman Creek near Virginia Dale and in Rist Canyon near Bellvue had recurrence intervals of 100 years and 16 years, respectively. At the gaging station on the Cache la Poudre River at the canyon mouth near Fort Collins (Site 34), the recurrence interval of the flood was computed to be 16 years. 70 FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO 1 600 l I I | 1200 — 1000 l 800 - 600 — 400h DISCHARGE, IN CUBIC FEET PER SECOND 200 - l 2400 | 1200 AUGUST 2 1976 I 2400 l 0 1200 AUGUST 1 l 1200 AUGUST 3 2400 FIGURE 56.—Discharge hydrograph for the Cache la Poudre River at mouth, near Greeley (Site 37). THE AFTERMATH The major brunt of the flood lasted only a few hours but during that short period of time an appalling amount of death and destruction occurred. Search and rescue operations were quickly begun and on August 2 Larimer County was declared a disaster area by the President of the United States. On the same day, a 6-month moratorium on construction in the flooded area along the Big Thompson River was declared by the county commissioners. This moratorium was ef- fected to restrict new construction or repairs to struc- tures damaged 50 percent or more until completion of a study to delineate the limits of the 100-year flood plain (Gingery Associates, Inc., 1976). THE HUMAN ELEMENT There were 139 fatalities, with 5 persons remaining on the list of missing. The ages of the victims ranged from 2 years to 94 years, with about 40 percent of them under 30 years of age and 28 percent over 60 years old. About two-thirds of the victims were residents of Col- orado; the other one-third were from 17 other States and the Philippine Islands. THE DAMAGE The dawn of Centennial Sunday unveiled an almost incomprehensible scene of destruction in many parts of the flooded area. Eastward along the Big Thompson River from Estes Park to Loveland Heights there was evidence of flooding but damage was not severe. At Loveland Heights, conditions changed dramatically, for, from this point downstream to Loveland, the scene was one of almost total devastation. As one observer described it, “* * * The scene was one to rival a com- bination tornado, flood, and earthquake * * *.” Condi- tions were similar along the North Fork Big Thompson River in the vicinity of Glen Haven. Downstream from Glen Haven, the major damage occurred to the county highway which generally parallels the river to Drake but several buildings located close to the river were severely damaged. Damage in Weld County along the downstream reach of the Big Thompson River was not severe, consisting mainly of bridge damage and debris accumulation. In the Cache la Poudre River basin, damage also was light, being mostly related to irriga- tion structures and partially completed flood-detention structures on Boxelder Creek. Although the estimates are far from complete, the total damage to date (1977) is estimated at $35.5 million. A summary of damage estimates compiled by the US. Army Corps of Engineers is given in table 5. Almost one-half of the total damage was related to rebuilding a major portion of US. Highway 34 which parallels the Big Thompson River from Loveland to Estes Park. As illustrated in figure 57, the highway embankment was completely destroyed in some reaches, leaving the canyon in a natural-appearing con- dition. Structural damages totaling almost $9 million included more than 50 percent damage to 252 struc- tures and less than 50 percent damage to 242 struc- tures. The aerial photographs shown in figure 58 vivid- ly illustrate structure damage at Drake. Figure 58A, made on August 29, 1973, represents approximate con- ditions of development at Drake before the flood. Figure 583 was made on August 3, 1976, after the flood, and figure 580 was made on October 29, 197 6, after the Drake area was cleared of debris and con- demned structures. The damage estimate also included $2.5 million for the Big Thompson dam, pipeline, and powerplant. The flood destroyed 438 automobiles and caused nearly $1 million in additional damages to house trailers. Several views of destroyed automobiles, some battered into almost unrecognizable shapes by debris and boulders while others almost buried by sediment are shown in METEOROLOGY, HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS 71 TABLE 5.—Damage estimates for the July 31-August 1, 1976 flood [Adapted from US. Army Corps of Engineers data] Breakdown Dollar Estimate LARIMER COUNTY Governmental clean-up operations ...... $ 1,611,000 Emergency efforts ..................... 656,700 Transportation damages ............... 17,420,000 Land-erosion damages ................. No dollar estimate Structural damages .................... 8,928,500 Personal property damages ............. 5,036,000 Employment losses .................... 115,500 Damages to public facilities not listed elsewhere ........................... 1,634,000 Indirect damages ...................... N 0 dollar estimate Emergency social assistance ............ 96,400 Total estimated economic and financial loses, LarhnerCounty ................ $ 135,498,100 WELD COUNTY Damage (preliminary—all public) ........ $ 45,000 Total estimated damage .......... $ 35,543,100 1Of the total losses, public damage is estimated at $23,671,600, and private damage is estimated at $11,826.500. figure 59. Another significant item was the $1.6 million expenditure for removal of more than 400,000 cubic yards of debris from the flooded area. No estimates were available for items, such as business losses from reduced tourist trade, loss of tax receipts, damage to wells and septic systems, and property devaluation because of location in a designated flood plain. COMPARISON WITH PREVIOUS RAIN STORMS AND F LOODS The rainfall and flood discharges that occurred on July 31—August 1, 1976, were unusually large, but they are not unprecedented for areas along the eastern foothills and plains of Colorado. A comparison between rainfalls for the 1976 storm and some previously recorded amounts in the area is shown in figure 60. Although many of the plotted points represent observ- ed measurements for the May 1935 and June 1965 storms, several other storms have produced rainfalls larger than those for the storm of July 31—August 1, 1976. Peak discharges for the July 31—August 1 flood at selected sites are plotted in figure 61 along with previ- ously recorded peak discharges at other locations. The 1976 peak discharges were greatlyexceeded by the May 1935 and June 1965 floods which occurred along the eastern plains and have been approximately equalled by several other floods some of which occurred in the eastern foothills. Only for drainage areas less than 4 square miles do the 1976 values exceed previously observed floods in eastern Colorado, and this may be largely attributed to failure to obtain flood data for small basins during previous extreme floods. Also shown in figure 61 is a plot of maximum observed flood discharges in the United States as developed by Mat- thai (1969). Again, the comparison shows that larger floods have been experienced at other locations in the United States. SOURCES OF DATA Sources of data, including both data in this report and supplementary or auxiliary data, are listed below. These sources were identified and listed in this report through the suggestions and efforts of the US. Water Resources Council, Hydrology Committee. 1. US. Geological Survey, Water Resources Division, Box 25046, Mail Stop 415, Denver Federal Center, Lakewood, Colorado 80225. Flood stages, discharges, volumes, travel times, flood profiles, hydrographs, and cross sections. 2. Colorado Water Conservation Board, 1313 Sherman Street, Denver, Colorado 80203. Flood stages, flood areas, discharges, volumes, travel times, flood pro- files, hydrographs, and cross sections. 3. Colorado Division of Water Resources, Room 802, Building A, New State Building, 1313 Sherman Street, Denver, Colorado 80203. Flood stages, discharges, volumes, travel times, flood areas, flood profiles, hydrographs, and cross sections. 4. Board of County Commissioners, Larimer County, Colorado. PO. Box 1190, Fort Collins, Colorado 80522. General hydrologic information and specific information on flood fatalities, flood damage, and rescue operations. 5. National Climatic Center, Federal Building, Ashe- ville, North Carolina 28801. Archives of all meteorological and hydrological data gathered by National Oceanic and Atmospheric Administration. 6. National Weather Service, Office of Hydrology, Silver Spring, Maryland 20910. Original bucket- survey data of rainfall. 72 FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER. COLORADO FIGURE 57.—Erosion damage to US. Highway 34 and to irrigation siphon at the gaging station on the Big Thompson River at mouth of canyon, near Drake (Site 23). 7. National Oceanic and Atmospheric Administration, Environmental Research Laboratory, Boulder, Col- orado 80302. Report by R. A. Maddox and others, “Meteorological Aspects of Big Thompson Flash Flood of July 31, 1976.” 8. National Hail Research Experiment, National Cen- ter for Atmospheric Research, Boulder, Colorado 80302. Radar data from NHRE Radar at Grover, 0010., and upper-air sounding data at Sterling, C010. 9. National Oceanic and Atmospheric Administration, Environmental Research Laboratories, Wave Propagation Laboratory, Boulder, Colorado 80302. Acoustic echo-sounder data at Table Mountain, Colo., on July 31, 1976. 10. Omaha District, US. Army Corps of Engineers, 6014 US. Post Office and Courthouse, 215 North 17th Street, Omaha, Nebraska 68102. Flood-damage data. 11. Colorado Water Conservation Board, 1313 Sherman Street, Denver, Colorado 80203. “Big Thompson River Flood of July 31—August 1, 1976, Larimer County, Colorado, Report of 1976.” 12. Bureau of Reclamation, Lower Missouri Region, PO. Box 25247, Denver Federal Center, Denver, Col- orado 80225. Contact Regional Director’s Office for specific information that is available. GAGING-STATION AND MISCELLANEOUS-SITE DATA PLATTE RIVER BASIN SITE 1: 06733000 BIG THOMPSON RIVER AT ESTES PARK, COLO. Location—Lat 40°22'42", long 105°30'48", in NW%NW% sec. 30, T. 5 N., R. 72 W., Larimer County, on right bank in Estes Park, 600 ft downstream from bridge on State Highways 7 and 66, 900 ft downstream from Black Canyon Creek, and 0.3 mi northeast of Estes Powerplant. Station is upstream from Lake Estes. METEOROLOGY, HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS 73 Drainage area. —137 mi?. Gage-height record—Water-stage recorder graph furnished by Colorado Division of Water Resources. Datum of gage is 7,492.5 ft above mean sea level (levels by US. Bureau of Reclamation). Discharge record.—Stage-discharge relation defined by current- meter measurement below 1,500 fth. Maxima.—July—August 1976: Discharge, 457 fti‘ls 2130 MDT July 31 (gage height, 3.64 ft). 1946 to June 1976: Discharge, 1,660 ft3/s June 18, 1949 (gage height, 3.16 ft site and datum then in use); gage height, 6.89 ft June 17,1965. Gage height and discharge at indicated time, 1976 Gage Gage Time height Discharge Time height Discharge (ft) (ft’ls) (ft) (ft’ls) July 30 August 1 00 .......... 1.84 153 0030 .......... 2.31 221 July 31 0100 .......... 2.38 232 0600 .......... 1.86 156 0200 .......... 2.14 195 1200 .......... 1.85 155 0230 .......... 2.10 190 1800 .......... 1.80 148 0300 .......... 2.75 292 1900 .......... 1.79 147 1930 .......... 2.20 204 0400 .......... 2.38 232 0500 .......... 2.47 246 2000 .......... 1.97 171 0600 .......... 2.39 233 2100 .......... 2.56 260 1200 .......... 2.23 209 2130 .......... 3.64 457 1700 .......... 2.22 207 2200 .......... 2.88 314 1800 .......... 2.30 219 2230 .......... 2.93 323 1820 .......... 2.36 228 2300 .......... 2.83 305 2000 .......... 2.29 218 2400 .......... 2.46 244 . 2400 .......... 2.34 225 SITE 2: 06734500 FISH CREEK NEAR ESTES PARK, COLO. Location. —Lat 40°22'10", long 105°29'40", in SW% sec. 29, T. 5 N., R. 72 W., Larimer County, on right bank 100 ft upstream from highwater line of Lake Estes, 0.4 mi upstream from bridge on US. Highway 36, and 2 mi southeast of Estes Park. Drainage area. —-16.0 miz. Gage-height record.—Water-stage recorder graph furnished by Colorado Division of Water Resources. Datum of gage is 7,475.80 ft above mean sea level (levels by US. Bureau of Reclamation). Discharge record.—Stage-discharge relation defined by standard Parshall flume rating and by slope-area measurement at 1,480 ft3/s. Maximo. —July—August 1976: Discharge, 182 ft3/s 2150 MDT July 31 (gage height, 4.02 ft). 1947 to June 1976: Discharge, 1,480 ft3/s May 25. 1951 (gage height, 7.32 ft from floodmark) caused by dam failure at Lily Lake. FIGURE 58.—Photographs showing aerial views of Drake: A, pre- flood conditions, August 29, 1973; B, immediately after the flood, August 3, 1976; C, after cleanup operations, October 29, 1976. 74 Gage height and discharge at indicated time, 1976 FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO FIGURE 59.—Views of destroyed automobiles. Gage Gage Time height Discharge Time height Discharge (ft) (ft‘ls) (ft) (ftJ/s) July 30 August 1 .......... 0.10 0.44 0100.......... 0.88 16 July 31 0220 .......... .78 13 0600 .......... 0.10 .44 0300 .......... .85 15 1200 .......... 0.11 .52 0400 .......... 3.16 123 1800 .......... 0.10 .44 0415 .......... 2.57 88 1920 .......... 0.10 .44 1930 .......... .21 1.6 0430 .......... 2.50 84 0500 .......... 1.58 40 2000 .......... .19 1.3 0600 .......... 1.02 20 2030 .......... .74 12 0800 .......... .71 11 2100 .......... 2.68 95 0900 .......... .74 12 2120 .......... 3.87 170 2130 .......... 3.89 172 1000 .......... .74 12 1 100 .......... .63 9.1 2150 .......... 4.02 182 1820 .......... .63 9.1 2200 .......... 3.97 178 1900 .......... 1.47 36 2220 .......... 2.96 111 2000 .......... 1.37 32 2230 .......... 2.86 105 2300 .......... 1.00 19 2300 .......... 1.75 48 2400 .......... .95 18 2400 .......... 1.10 23 SITE 3: 06735500 BIG THOMPSON RIVER NEAR ESTES PARK, COLO. Location—Lat 40°22'35", long 105°29'06", in NE%NE% sec. 29, T. 5 N., R. 72 W., Larimer County, on right bank 100 ft upstream from Dry Gulch, 600 ft downstream from Olympus Dam, and 2.0 mi east of Estes Park. Drainage tired—155 miz. Gage-height record.—Water-stage recorder graph until midnight July 31 at which time intakes were plugged. Record furnished by Colorado Division of Water Resources. Datum of gage is 7 ,422.5 ft above mean sea level (levels by US. Bureau of Reclamation). Discharge record.——Gates at Olympus Dam were closed at 2055 MDT July 31. Subsequent stream discharge which derived from Dry Gulch caused the Parshall flume to be covered by sediment making the stage-discharge relation indefinite. Maxima.—July—August 1976: Discharge, undetermined; gage height (5.51 ft) 2230 MDT July 31 (backwater from Dry Gulch). 1930 to June 1976: Discharge, 2.800 ft3/s June 20. 1933 (gage height, 4.0 ft, site and datum then in use). METEOROLOGY, HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS 75 25 I | * * * 20 — * '- * * * i3 15 ~ * * - :I: o E Z i E * 3‘: 10 ~ I _ * * * l * * 515 * l I July 31—August 1, 1976, rainfall * * * a: _ ' * * * * * Prevuous|y observed rainfall 5 — an?" * * * _ * I * * * l 1 o o 5 1o 15 RAINFALL DURATION, IN HOURS FIGURE 60.—Comparison between July 31—August 1, 1976, rainfalls and previously observed rainfalls in eastern Colorado. Gage height at indicated time, 1976 Maximum—July-Aug'ust 1976: Discharge, 3,210 ft3/s about 2215 Gage height Gage height MDT Jilly 31- Time 1m Time (m 00 July 30 1 40 2105“” 31‘0”““9‘1 1 73 SITE 5: DRY GULCH AT ESTES PARK, COLO. I'IfdinéI """" I 2130 I I I I I I I I I I I I I I II 3I45 (MISCELLANEOUS SITE) 0600 ................ 1.40 2150 ................ 4.95 1200 ................ 1.39 2200 ................ 5.40 Location—Lat 40°22'42", long 105°29'15”, in NE l/4NWl/4 sec. 29, 1800 ................ 1.33 2210 ................ 5.50 T. 5 N., R. 72 w” Larimer County, 1000 ft upstream from mouth, i392 """""""" 1:28 2245 ............... 5.51 0.9 mi east of Estes Park, at US. Highway 34 bridge over Dry 2300 ................ 4.95 Gulch. 1935 ................ 1.46 2315 ................ 2.55 Drainage area,—6.12 11117, éggg """""""" 1:: 2400 """ AuguIsItl ‘ 1'34 Discharge record—Peak discharge by computation of flow through 2030 I I I I I I I I I I I I I I II 1I6O Gage intakes culvert . 2055 ................ 2.02 plugged Maximum—July—August 1976: Discharge. 4,460 fta/s about 2230 SITE 4: DRY GULCH NEAR ESTES PARK, COLo. (MISCELLANEOUS SITE) Location.—Lat 40°24’22", long 105°28'37", in NEMINE‘A sec. 17, T. 5 N., R. 72 W.. Larimer County, 2.2 mi upstream from US. Highway 34, and 2.2 mi northeast of Estes Park. Drainage area—2.00 miz. Discharge record—Peak discharge by slope-area measurement. MDT July 31. SITE 6: BIG THOMPSON RIVER BELOW ESTES PARK, COLO. (MISCELLANEOUS SITE) Location—Lat 40°22'59", long 105°28'11", in NE’ASW‘A sec. 21, T. 5 N ., R. 72 W., Larimer County. 0.4 mi upstream from Loveland Heights, 1.2 mi downstream from Olympus Dam, and 2 mi east of Estes Park. 76 FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER. COLORADO 1,000,000 _ I l I I I I l l I T I I I I I l I I I I I I I I I I I I I I I l I I I I I I I I a + Maximum floods in eastern Colorado + j :1: Flood of July 31—August 1, 1976 ‘ _ + . O ” _ Z o 0 Lu LO ,1 100,000 _— — Hi — I )— _ _ a 1 + ' ,L _ _ 2 - ++ ED 3 U " - Z w. _ _ 2 < + a + + 10,000 — — g : 1 + IT” + + * ‘ x — * +* : < _ E - * + + I _ + + * :1: _ _ :1: *+ * - * + * * _ + ** _ 1000 I I I I I I | I I L I I l l l I I I I I I I I I I I I I I I l I I l I I I I | l l I I l 0.1 1 10 100 1000 10,000 DRAINAGE AREA, IN SQUARE MILES FIGURE 61.—Relation of peak discharge to drainage area for flood of July 31—August 1, 1976, and previous maximum floods. Drainage area—164 mii. Discharge record—Peak discharge by slope-area measurement. Maximum.——July~August 1976: Discharge, 4,330 ft3/s about 2300 MDT July 31. SITE 7: BIG THOMPSON RIVER TRIBUTARY BELOW LOVELAND HEIGHTS, COLO. (MISCELLANEOUS SITE) Location—Lat 40°23'44'1 long 105°27'34", in SE‘ASE‘A sec. 16, T. 5 N ., R. 72 W., Larimer County, 0.4 mi upstream from mouth, 0.5 mi northeast of Loveland Heights, and 2.5 mi east of Estes Park. Drainage area—1.37 miY. Discharge record—Peak discharge by slope-area measurement. Maximum—July—August 1976: Discharge, 8,700 ft3/s July 31. SITE 8: DARK GULCH AT GLEN COMFORT, COLO. (MISCELLANEOUS SITE) Location—Lat 40°23'44", long 105°26’17”, in SW% sec. 14, T. 5 N., R. 72 W. (unsurveyed), Larimer County, 800 ft upstream from mouth, 800 ft north of Glen Comfort, and 3.5 mi east of Estes Park. Drainage area—1.00 mi2. Discharge record—Peak discharge by slope-area measurement. Maximum. —Ju1y—August 1976: Discharge, 7,210 ft’ls July 31. SITE 9: NOELS DRAW AT GLEN COMFORT. COLO. (MISCELLANEOUS SITE) Location-Lat 40°23’25", long 105°26'00”, in NE1/4NW1A sec. 23, T. 5 N., R. 72 W., Larimer County, 1,100 ft upstream from mouth, 0.3 mi south of Glen Comfort, and 3.8 mi east of Estes Park. Drainage area—3.37 H112. Discharge record—Peak discharge by slope-area measurement. Maximum.—July—August 1976: Discharge, 6,910 fth July 31. SITE 10: RABBIT GULCH NEAR DRAKE, COLO. (MISCELLANEOUS SITE) Location—Lat 40°24'23”, long 105°24'17", in NE‘ANE‘A sec. 13, T. 5 N ., R. 72 W. (unsurveyed), Larimer County, 300 ft upstream from mouth, 5.5 mi northeast of Estes Park, and 4.0 mi southwest of Drake. Drainage area—3.41 mi“. Discharge record—Peak discharge by slope-area measurement. Maximum—July—August 1976: Discharge, 3,540 ft3/s July 31. METEOROLOGY, HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS 77 SITE 11: LONG GULCH NEAR DRAKE, COLO. (MISCELLANEOUS SITE) Location—Lat 40°24'46", long 105°24'04", in SW% sec. 7, T. 5 N ., R. 71 W., Larimer County, 1,200 ft upstream from mouth, 5.8 mi northeast of Estes Park, and 3.7 mi southwest of Drake. Drainage area—1.99 miz. Discharge record—Peak discharge by slope-area measurement. Maximum—July—August 1976: Discharge, 5,500 ft3/s July 31. SITE 12: BIG THOMPSON RIVER ABOVE DRAKE, COLO. (MISCELLANEOUS SITE) Location—Lat 40°25'39", long 105°20'37", in SW% sec. 3, T. 5 N., R. 71 W., Larimer County, 0.66 mi upstream of confluence with North Fork Big Thompson River in Drake, and 9 mi northeast of Estes Park. Drainage area—189 mi”. Discharge record—Peak discharge by slope-area measurement. Maximum—July—August 1976: Discharge, 28,200 ftfl/s about 2100 MDT July 31. SITE 13: NORTH FORK BIG THOMPSON RIVER AT GLEN HAVEN, COLO. (MISCELLANEOUS SITE) Location—Lat 40°27'17", long 105°27’05", in NE‘ASW‘A sec. 27, T. 6 N., R. 72 W., Larimer County, 0.1 mi upstream from Fox Creek, 0.2 mi northwest of Glen Haven, and 5.7 mi northeast of Estes Park. Drainage area—18.5 miz. Discharge record—Peak discharge by slope-area measurement. Maximum-July—August 1976: Discharge, 888 ft3/s July 31. SITE 14: FOX CREEK AT GLEN HAVEN, COLO. (MISCELLANEOUS SITE) Location—Lat 40°27’17", long 105°27’13", in NEMISW‘A sec. 27, T. 6 N ., R. 72 W., Larimer County, 0.2 mi upstream from mouth, 0.3 mi west of Glen Haven, and 5.7 mi northeast of Estes Park. Drainage area—7.18 mi“. Discharge record—Peak discharge by slope-area measurement. Maximum—July-August 1976: Discharge, 1,300 fth July 31. SITE 15: WEST CREEK NEAR GLEN HAVEN, COLO. (MISCELLANEOUS SITE) Location—Lat 40°26'32", long 105°27’40", in SE1/4 sec. 33, T. 6 N., R. 72 W. (unsurveyed), Larimer County, 0.2 mi upstream from mouth, 1.0 mi southwest of Glen Haven, and 4.6 mi northeast of Estes Park. Drainage area. —23. 1 miz. Discharge record—Peak discharge by slope-area measurement. Maximum. —-July—August 1976: Discharge 2,320 ft3/s July 31. SITE 16: DEVILS GULCH NEAR GLEN HAVEN, COLO. (MISCELLANEOUS SITE) Location—Lat 40°26'24", long 105°27'31", in SE1/4 sec. 33, T. 6 N., R. 72 W. (unsurveyed), Larimer County, 600 ft upstream from mouth, 1.1 mi south of Glen Haven, and 4.5 mi northeast of Estes Park. Drainage area—0.91 miz. Discharge record—Peak discharge by slope-area measurement. Maximum—July—August 1976: Discharge, 2,810 ftals July 31. SITE 17: NORTH FORK BIG THOMPSON RIVER TRIBUTARY NEAR GLEN HAVEN, COLO. (MISCELLANEOUS SITE) Location—Lat 40°27'14”, long 105°26’04", in NW‘ASEMI sec. 26, T. 6 N ., R. 72 W., Larimer County, 300 ft upstream from mouth, 0.8 mi east of Glen Haven, and 5.7 mi northeast of Estes Park. Drainage area—1.38 miz. Discharge record—Peak discharge by slope-area measurement. Maximum. —July—August 1976: Discharge, 9,670 ft3/s July 31. SITE 18: BLACK CREEK NEAR GLEN HAVEN, COLO. (MISCELLANEOUS SITE) Location—Lat 40°28’04", long 105°25'28", in SE% sec. 23, T. 6 N., R. 72 W. (unsurveyed), Larimer County, 0.2 mi upstream from mouth, 1.6 mi northeast of Glen Haven, and 7 mi northeast of Estes Park. Drainage area—3.17 mi2. Discharge record—Peak discharge by slope-area measurement. Maximum.—July—August 1976: Discharge, 1,990 ft’ls about 2300 MDT July 31. SITE 19: MILLER FORK NEAR GLEN HAVEN, COLO. (MISCELLANEOUS SITE) Location—Lat 40°27'47", long 105°25'13", in NW%NW‘/4 sec. 25, T. 6 N ., R. 72 W., Larimer County, 0.3 mi downstream from Black Creek, 0.4 mi upstream from mouth, 1.6 mi east of Glen Haven, and 7.0 mi northeast of Estes Park. Drainage area—13.9 mix. Discharge record—Peak discharge by slope-area measurement. Maximum. —July—August 1976: Discharge, 2,060 fta/s about 2300 MDT on July 31. SITE 20: NORTH FORK BIG THOMPSON RIVER TRIBUTARY NEAR DRAKE, COLO. (MISCELLANEOUS SITE) Location—Lat 40°26’55", long 105°24’11", in NW‘/4 sec. 31, T. 6 N ., R. 71 W. (unsurveyed), Larimer County, 0.8 mi upstream from mouth, 2.4 mi east of Glen Haven, and 3.5 mi northwest of Drake. Drainage area—1.26 miz. Discharge record—Peak discharge by slope-area measurement. Maximum. —July—August 1976: Discharge, 3.240 ft3/s July 31. SITE 21: 06736000 NORTH FORK BIG THOMPSON RIVER AT DRAKE, COLO. Location. —Lat 40°26'00", long 105°20'18", in NW% sec. 3, T. 5 N., R. 71 W., Larimer County, 400 ft upstream from mouth at Drake. Drainage area—85.1 mi‘. Gage-height record.—Water-stage recorder graph furnished by Colorado Division of Water Resources. Stage record unusable after 2300 MDT, July 31, because gage intakes were plugged by sediment. Altitude of gage is 6,170 ft from topographic map. Discharge record.—Stage-discharge relation extended above 1,580 ft3/s on basis of slope-area measurement made at Site 1.6 mi upstream. 78 FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO Maxima.—July—August 1976: Discharge, 8,710 ft3/s 2140 MDT July 31 (gage height, 9.21 ft). 1947 to June 1976: Discharge, 1,290 ft3/s June 16, 1965 (gage height, 5.66 ft). Gage height and discharge at indicated time, 1976 Gage Gage Time height Discharge Time height Discharge (ft) (ft’ls) (ft) (ft’ls) July 3 July 31—Continued .......... 3.78 42 2030 . . . . . . . . . . 5.30 925 July 31 2050 .......... 7.00 3,000 0600 .......... 3.74 38 2100 .......... 8.40 6,000 1200 .......... 3.72 36 2110 .......... 8.66 6,780 1600 .......... 3.71 35 2135 .......... 8.38 5,940 1700 .......... 3.72 36 1745 .......... 3.77 41 2140 .......... 9.21 8,710 2150 .......... 9.09 8,260 1820 .......... 3.76 40 2210 .......... 9.08 8,230 1855 .......... 3.80 44 2245 .......... 8.24 5,520 1900 .......... 4.17 132 2300 .......... 8.05 5,100 1910 .......... 4.65 440 Gage intakes plugged about 1950 .......... 5.03 704 2300 on July 31. 2025 .......... 5.14 792 SITE 22: BIG THOMPSON RIVER BELOW DRAKE, COLO. (MISCELfANEOUS SITE) Location—Lat 40°25'52", long 105°19'37", in NE1/4 sec. 3, T. 5 N., R. 71 W. (unsurveyed), 0.6 mi downstream from North Fork Big Thompson River, 0.6 mi east of Drake, and 3.4 mi northwest of Cedar Cove. Drainage area—276 miz. Discharge record—Peak discharge by slope-area measurement. Maximum—July—August 1976: Discharge, 30,100 ft3/S July 31. SITE 23: 06738000 BIG THOMPSON RIVER AT MOUTH OF CANYON, NEAR DRAKE, COLO. Location—Lat 40°25'18", long 105°13’34", in SWl/ASWM: sec. 3, T. 5 N., R. 70 W., Larimer County, on right bank at mouth of can- yon, 400 ft upstream from Handy Ditch diversion dam, and 6.0 mi east of Drake. Drainage area. ~—305 miY. Gage-height record. —Water-stage recorder graph furnished by Colorado Division of Water Resources. Gage was destroyed by flood; but one observed stage was obtained at 21 10 MDT, and the peak stage which occurred at 2140 MDT was obtained by leveling to high-water marks. Datum of gage is 5,297.47 ft above mean sea level (levels by US. Bureau of Reclamation). Discharge record.—Stage-discharge relation extended above 2,310 ft3/s on basis of slope-area measurement. Maxima.—July—August 1976: Discharge, 31,200 ft3/s 2140 MDT July 31 (gage height, 19.7 ft from floodmarks). 1887-92, 1895—1903, 1926—33, 1938—49, 1951 to June 1976: Discharge, 7,600 ft3/s July 19, 1945 (gage height, 7.55 ft, site and datum then in use). A peak discharge of 8,000 ft3/s was recorded at a site 5 mi up stream on July 31, 1919. Gage height and discharge at indicated time, 1976 Gage Gage Time height Discharge Time height Discharge (ft) (ft’ls) (ft) (ftJ/s) July July 31 —Continued .......... .76 104 2015.......... 1.74 99 July 31 2050 .......... 1.76 104 0600 .......... .72 95 2110 .......... 9.0 5,500 1200 .......... 1.69 89 2140 .......... 19.7 31,200 1800 .......... 1.68 87 Gage destroyed 1930 .......... 1 .68 87 SITE 24: BIG THOMPSON RIVER BELOW GREEN RIDGE GLADE, NEAR LOVELAND, COLO. (MISCELLANEOUS SITE) Location—Lat 40°25'05", long 105°12'02", in NW‘ANE‘A sec. 11, T. 5 N., R. 70 W., Larimer County, 2,300 ft downstream from mouth of Green Ridge Glade, 2,200 ft upstream from bridge on US. Highway 34, and 6.5 mi west of Loveland. Drainage area—311 miz. Discharge record—Peak discharge by slope-area measurement. Maximum—July—August 1976: Discharge, 27,000 ft3/s July 31. SITE 25: REDSTONE CREEK NEAR MASONVILLE, COLO. (MISCELLANEOUS SITE) Location—Lat 40°30'19”, long 105°11'49", in NW%NE% sec. 11, T. 6 N ., R. 70 W., Larimer County, 50 ft downstream from Hansen Canal crossing, 3.4 mi southwest of Spring Canyon Dam, and 1.5 mi northeast of Masonville. Drainage area—29.1 mi”. Discharge record—Peak discharge by slope-area measurement. Maximum. —July—August 1976: Discharge, 2,640 ft3/s August 1. SITE 26: 06739500 BUCKHORN CREEK NEAR MASONVILLE, COLO. (DISCONTINUED GAGING-STATION SITE) Location—Lat 40°27'15", long 105°11’50", in SE% sec. 26, T. 6 N., R. 70 W., Larimer County, on right bank 1.5 mi upstream from Buckhorn Reservoir Dam and 2.5 mi south of Masonville. Drainage area—131 mix. Gage-height record—Station at site discontinued but water-stage recorder graph for site 1.7 mi downstream furnished by Colorado Division of Water Resources. Discharge record.—Stage-discharge relation defined by current- meter measurements below 1,300 ft3/s and extended on basis of two slope-area measurements. Maxima—July—August 1976: Discharge, 3,400 ft’ls August 1 (gage height, 8.1 ft from floodmarks). 1923, 1938, 1947—June 1976: Discharge, 14,000 ft3/s August 3, 1951 (gage height, 13.40 ft). METEOROLOGY, HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS 79 SITE 27: LITTLE THOMPSON RIVER NEAR ESTES PARK, COLO. (MISCELLANEOUS SITE) Location—Lat 40°20'06", long 105°25'48", in SW‘ASW1/4 sec. 2, T. 4 N ., R. 72 W., Larimer County, 900 ft upstream from mouth of Big Gulch, 0.35 mi south of Meadowdale Ranch, and 5.1 mi southeast of Estes Park. Drainage area—2.77 miz. Discharge record—Peak discharge by slope-area measurement. Maximum—July—August 1976: Discharge, 1,940 fta/s 2130 MDT July 31. SITE 28: 06744000 BIG THOMPSON RIVER AT MOUTH, NEAR LASALLE, COLO. Location—Lat 40°21’00", long 104°47'04", in SWIASEIA sec. 33, T. 5 N ., R. 66 W., Weld County, on left bank just southeast of gage on Evans Town ditch, 0.7 mi upstream from highway bridge, 1.6 mi upstream from mouth, and 4.2 mi west of LaSalle. Drainage area—828 mi7. Gage-height record.—Water-stage recorder graph furnished by Colorado Division of Water Resources. Altitude of gage is 4,680 ft from topographic map. Discharge record.—Stage-discharge relation defined by current- meter measurements. Maxima.—July—August 1976: Discharge, 2,470 ft3/s 2235 MDT August 1 (gage height, 7.82 ft). 1914—15, 1927 to June 1976: Discharge, 6,100 ft3/s August 4, 1951 (gage height, 7.80 ft at site 0.7 mi downstream at different datum); gage height, 8.70 ft May 9, 1957, present datum. Gage height and discharge at indicated time, 1976 Gage Gage Time height Discharge Time height Discharge (m (ft’ls) (ft) (ftJ/s) July 30 August 1 —Continued 00 ....... 1.61 77 1300 ....... 3.45 559 July 31 1400 ....... 4.03 798 1500 ....... 4.50 1,000 0600 ....... 152 78 1600 ....... 4.93 1,180 1200 _______ 1.62 78 1700 ....... 5.50 1,410 1800 ....... 1.66 85 2340 ,,,,,,, 1.62 78 1800 ....... 5.99 1,620 2400 ....... 1.79 109 1900 ------- 6.42 1.790 2000 ....... 7.03 2,030 August 1 2100 ....... 7.45 2,240 2200 ....... 7.74 2,410 0030 ....... 1.75 102 0600 ....... 1.72 96 2235 ....... 7.82 2,470 0735 ....... 1_71 94 2300 ....... 7.80 2.460 0320 _______ 1.55 67 2400 ....... 7.66 2,360 0900 ....... 1.66 85 August 2 0920 ....... 1.85 120 0100 ....... 7.34 2,180 . .. 1.93 135 0200 ...... 6.99 2,010 2.17 184 0600 ....... 4.89 1,170 2.79 344 0800 ...... 4.52 1,010 1200 ....... 3.08 430 0840 ....... 4.43 965 Gage height and discharge at indicated time, 1976 —Continued Gage Gage Time height Discharge Time height Discharge (ft) (ft’ls) (ft) (ft’ls) August 2—Continued A ugust 3 1000 ....... 4.45 975 0100 ....... 5.00 1,220 1020 ....... 4.49 995 0200 ....... 5.06 1,240 1100 ....... 4.49 995 0300 ....... 5.05 1,240 1200 ....... 4.48 990 0400 ....... 4.99 1,210 1700 ....... 4.01 789 0600 ....... 4.67 1,080 0800 ....... 4.50 1,020 1800 ....... 4.09 825 4.29 980 2100 ....... 4.38 946 3.77 740 2400 ....... 4.86 1,160 3,60 667 2400 ....... 3.12 472 SITE 29: DALE CREEK TRIBUTARY NEAR VIRGINIA DALE, COLO. (MISCELLANEOUS SITE) Location—Lat 40°57'36", long 105°21'39", in NW‘ANWMI sec. 4, T. 11 N ., R. 71 W., Larimer County, 300 ft upstream from mouth, at culvert on U.S. Highway 287, and 0.75 mi northwest of Virginia Dale. Drainage area—0.68 miY. Discharge record—Peak discharge by computation of flow through culvert. Maximum—July—August 1976: Discharge, 727 ft3/S July 31. SITE 30: DEADMAN CREEK NEAR VIRGINIA DALE, COLO. (MISCELLANEOUS SITE) Location—Lat 40°55'50", long 105°20'57”, in NE%NE% sec. 16, T. 11 N., R. 71 W., Larimer County, 1,700 ft upstream from mouth, 0.65 mi downstream from U.S. Highway 287, and 1.6 mi south of Virginia Dale. Drainage area—23.7 miY. Discharge record—Peak discharge by slope-area measurement. Maximum.—July—August 1976: Discharge, 7,400 ft3/s July 31. SITE 31: STONEWALL CREEK NEAR LIVERMORE, COLO. (MISCELLANEOUS SITE) Location. —Lat 40°48'37", long 105°15'04”, in NE‘ANEl/a sec. 29, T. 10 N., R. 70 W., Larimer County, 0.65 mi upstream from mouth, 1.6 mi downstream from North Poudre Ditch crossing, and 2.2 mi northwest of Livermore. Drainage area—31.9 mii. Discharge record—Peak discharge by slope-area measurement. Maximum—July-August 1976: Discharge, 3,470 ft3/S July 31. 80 FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO SITE 32: LONE PINE CREEK NEAR LIVERMORE, COLO. (MISCELLANEOUS SITE) Location—Lat 40°47'44", long 105°17'24”, in NW%NE‘/4 sec. 36, T. 10 N ., R. 71 W., Larimer County, 50 ft downstream from irriga- tion ditch diversion, 2.2 mi upstream from mouth, and 3.9 mi west of Livermore. Drainage area—86.3 mii. Discharge record—Peak discharge by slope-area measurement. Maximum—July—August 1976: Discharge, 2,590 ft3/s July 31. SITE 33: NORTH FORK CACHE LA POUDRE RIVER AT LIVERMORE, COLO. (DISCONTINUED GAGING—STATION SITE) Location—Lat 40°47’15", long 105°15'08", in SW‘ASEIA sec. 32, T. 10 N., R. 70 W., Larimer County, 1,000 ft upstream from bridge on State Highway 200, and 2.0 mi west of Livermore. Drainage area. —-539 miz. Discharge record—Peak discharge by slope-area measurement. Maxima. —July—August 1976: Discharge, 9,460 ft3/s July 31. 1904, 1929—31: Discharge, 20,000 ft3/S May 20. 1904. SITE 34: 06752000 CACHE LA POUDRE RIVER AT MOUTH OF CANYON, NEAR FORT COLLINS, COLO. Location—Lat 40°39’52", long 105°13'26", in NW% sec. 15, T. 8 N., R. 70 W., Larimer County, on left bank at month of canyon, 0.5 mi downstream from headgate of Poudre Valley Canal, 1.2 mi upstream from Lewstone Creek, and 9.3 mi northwest of courthouse in Fort Collins. Drainage area—1,056 miz. Gage-height record—Water-stage recorder graph furnished by Colorado Division of Water Resources. Altitude of gage is 5,220 ft from topographic map. Discharge record.—Stage-discharge relation defined by current- meter measurements to 3,800 fth and extended on basis of slope- area measurement. Maxima—July—August 1976: Discharge, 7,340 ft3/s 0130 MDT August 1 (gage height, 7.86 ft). 1882 to June 1976: Maximum discharge not determined, occur- red May 20, 1904; maximum discharge determined, 21,000 ft3/s June 9, 1891, caused by failure of Chambers Lake Dam. Gage height and discharge at indicated time, 1976 Gage Gage Gage Time height Discharge Time height Discharge (ft) (ft’ls) (ft) (ftJ/s) July 30 August 1 2400 ....... 2.35 252 0100 ....... 7.02 5,820 July 31 0130 ....... 7.86 7,340 0200 ....... 6.77 5,380 0500 ....... 2.34 248 0300 ....... 5.58 3,480 1000 ....... 2.35 252 0400 ...... 4.27 1,790 1200 ....... 2.38 266 1800 ....... 2.40 275 0600 ....... 3.53 1,070 2030 ....... 2.40 275 0800 ....... 3.29 872 1000 ....... 3.21 808 2150 ....... 2.45 300 1200 ....... 3.15 760 2210 ....... 2.45 300 1400 ....... 3.12 736 2300 ....... 2.68 428 2400 ....... 3.34 912 Gage height and discharge at indicated time, 1976—Continued Gage Gage Time height Discharge Time height Discharge (ft) (fta/s) (ft) (its/s) A ugust 1 —Continued A ugust 2—Continued 1600 ....... 3.10 720 0840 ....... 4.95 2,620 1830 ....... 3.09 712 0920 ....... 4.93 2,590 1900 ...... 3.10 720 1000 ....... 4.13 1,630 2000 ....... 3.20 800 1200 ....... 3.98 1.480 2200 ...... 3.75 1,260 1400 ....... 4.00 1,500 2330 ....... 3.91 1,410 1520 ------- 4-00 1,500 2400 ....... 3.92 1,420 1600 ....... 3.65 1,180 August2 1800 ....... 3.63 1,160 2000 ....... 3.62 1,150 0200 ....... 3.87 1,370 2200 ------- 3.61 1,140 0400 ....... 3.82 1,330 3.82 1.330 2315 ....... 3.22 816 3.83 1,340 2400 ....... 3.21 808 0800 ....... 3.83 1,340 SITE 35: RIST CANYON NEAR BELLVUE, COLO. (MISCELLANEOUS SITE) Location—Lat 40°37’43", long 105°12'44", in SE‘ASEIA sec. 27, T. 8 N., R. 70 W., Larimer County, 1,000 ft upstream from bridge on county road, and 2.0 mi west of Bellvue. Drainage area—5.27 mi2. Discharge record—Peak discharge by slope-area measurement. Maximum—July—August 1976: Discharge, 2,710 ft3/s 0030 MDT August 1. SITE 36: 06752260 CACHE LA POUDRE RIVER AT FORT COLLINS, COLO. Location—Lat 40°35’17", long 105°04'08", in NE1/4SW1A sec. 12, T. 7 N., R. 69 W., Larimer County, on left bank 150 ft downstream from Lincoln Avenue Bridge, and 2,200 ft east of intersection of College Avenue (U.S. Highway 287) and Mountain Avenue in Fort Collins. Drainage area—1,129 miz. Gage-height record.—Water-stage recorder graph. Altitude of gage is 4,940 ft from topographic map. Discharge record.—Stage-discharge relation defined to 1,250 ft3/s by current-meter measurements and extended on basis of slope- area measurement. Maxima.—July—August 1976: Discharge, 5,700 ft3/s 0430 MDT August 1 (gage height, 8.84 ft). April 1975 to June 1976: Discharge, 2,200 ft3/s June 19, 1975 (gage height, 5.93 ft). Gage height and discharge at indicated time, 1976 Gage Gage Time height Discharge Time height Discharge (ft) (ft’ls) (it) (ft’ls) July 30 July 31 —Continued ....... 2. 1 1 62 0600 ....... 2.00 44 July 31 0700 ....... 1.96 38 0800 ....... 1.94 35 0100 ...... 2.00 46 0900 ....... 1.95 36 0200 ....... 1.91 31 1000 ....... 1.95 36 0300 ....... 1 .90 30 0400 ....... 2.00 44 1100 ....... 1.96 38 0500 ....... 2.00 44 1200 ....... 2.04 50 METEOROLOGY, HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS Gage height and discharge at indicated time, 1976—Continued 81 Gage height and discharge at indicated time, 1976—Continued Gage Gage Time height Discharge Time height Discharge (ft) 1ft3/s) (ftl 1ft3/s) August 1—Continued August 2—Continued 0800 ....... 2.05 38 0800 ....... 4.26 660 0900 ....... 2.02 35 1230 ....... 2.09 42 1000 ....... 4.07 565 1130 ....... 3.99 529 1345 ....... 2.07 40 1200 ....... 4.00 534 1410 ....... 2.08 41 1400 ....... 4.66 926 1540 ....... 2.02 35 1600 ....... 5.03 1,120 1700 ....... 2.01 34 1800 ....... 4.02 560 1800 ....... 5.17 1,220 2000 ....... 5.18 1,230 1830 ....... 4.27 676 2200 ....... 5.18 1,230 1900 ....... 4.33 709 2400 ....... 5.26 1,290 2000 ....... 4.44 764 2100 ....... 4.64 872 August3 2200 ....... 4.82 980 0200 ....... 5.32 1,340 2215 ....... 5.40 1,400 . 5.31 1,330 2300 ....... 5.52 1,500 5.21 1,250 2400 ....... 5.58 1,550 4.77 950 August2 0800 ....... 4.38 726 0030 ...... 5.62 1,580 1200 ....... 3.92 502 0200 ...... 5.39 1,400 1600 ....... 3.62 380 0400 ....... 4.83 986 2000 ....... 3.42 311 0600 ...... 4.49 786 2400 ....... 3.30 272 SELECTED REFERENCES Follansbee, Robert, and Sawyer, L. R., 1948, Floods in Colorado: U.S. Geol. Survey Water-Supply Paper 997, 151 p. Gage Gage Time height Discharge Time height Discharge (ft! (its/s) (it! (its/s! July 31 —Continued August I—Continued 1300 ------- 2.02 47 2000 ....... 2.58 182 1400 ....... 2.02 47 2100 ....... 2.69 213 1500 ------- 2.02 47 2200 ....... 3.75 610 2300 ....... 4.59 1,050 1600 ------- 2.01 45 2400 ....... 4.61 1,060 1700 ....... 2.03 48 1800 ....... 2.07 55 August 2 1900 ....... 2.08 57 2000 ....... 2-09 59 0100 ....... 4.51 1,000 0200 ....... 4.43 956 2100 ------- 2.10 61 0300 ....... 4.37 920 2200 ------- 2.11 62 0400 ....... 4.26 854 2300 ....... 2.11 62 0500 _______ 4,23 836 2400 ....... 2.19 79 0600 ....... 4.22 836 Augustl 0700 ....... 4.38 926 0800 ....... 4.49 992 0100 ------- 2.38 126 0900 ....... 4.55 1,030 0200 ....... 2.30 105 1000 ,,,,,,, 4,39 932 0300 ....... 7.67 3.940 0400 ....... 8.35 4.920 1100 ....... 5.22 1,510 0430 ....... 8.84 5,700 1200 ....... 4,65 1,130 1300 ....... 4.19 878 0500 ....... 8.32 4.870 1400 ....... 4.11 836 0600 ....... 6.77 2.920 1500 ....... 3.87 735 0700 ....... 5.13 1,440 0800 ....... 4.54 1,020 1600 ,,,,,,, 3,92 755 0900 ....... 4.14 790 1700 ,,,,,,, 3,63 630 1000 ...... 3.87 665 1800 ....... 3.49 580 1100 ....... 3.47 485 1900 ....... 3.07 426 1200 ...... 3.53 510 2000 ....... 2.91 374 1300 ....... 3.3 5 434 1400 ....... 3.10 346 2100 ....... 2.82 346 2200 ....... 2.59 280 1500 ....... 3.00 310 2300 ....... 2.13 164 1600 ....... 2.66 206 2400 ....... 2.00 135 1700 ....... 2.45 141 1800 ....... 2.59 185 1900 ....... 2.58 182 SITE 37: 06752500 CACHE LA POUDRE RIVER NEAR GREELEY, COLO. Location—Lat 40°25’04", long 104°38’22”, in NW1/4 sec. 11, T. 5 N ., R. 65 W., Weld County, on right bank 25 ft downstream from highway bridge, 2.9 mi east of courthouse in Greeley, and 3.0 mi upstream from mouth. Drainage area. —1,877 mii. Gage-height record—Water-stage recorder graph furnished by Colorado Division of Water Resources. Altitude of gage is 4,610 ft from topographic map. Discharge record.—Stage-discharge relation defined by current- meter measurements. Maxima. —July—August 1976: Discharge, 1,600 ft3/s 0030 MDT August 2 (gage height, 5.62 ft). 1903—4, 1914—19, 1924 to June 1976: Daily discharge, 4,220 ft3/s June 24,26,1917. Gage height and discharge at indicated time, 1976' Gage Gage Time height Discharge Time height Discharge (m (ft’/sl 1m lie/s) July 31 August I ....... 1 .95 28 0 1 35 ....... 1 .98 3 1 O7 1 5 ....... 1 .93 2 7 Gingery Associates, Inc., 1976, Special flood plain information report, Big Thompson River and tributaries, Larimer County, Colorado: Englewood, Colo., prepared for Colorado Water Con- serv. Board, Federal Insurance Adm., and Larimer County, 0010., v. 1, 21 p. Grozier, R. U., McCain, J. F., Lang, L. F., and Merriman, D. C., 1976, The Big Thompson River flood of July 31—August 1, 1976, Larimer County, Colorado: Colorado Water Conserv. Board Flood Inf. Rept., 78 p. Lott, G. A., 1976, Precipitable water over the United States, Vol- ume 1. Monthly means: Natl. Oceanic Atmospheric Adm. Tech. Rept. NWS 20, 173 p. Matthai, H. F., 1969, Floods of June 1965 in South Platte River basin, Colorado: US. Geol. Survey Water-Supply Paper 1850—8, 64 p. McCain, J. F., and Jarrett, R. D., 1976, Manual for estimating flood characteristics of natural-flow streams in Colorado: Colorado Water Conserv. Board Tech. Manual 1, 68 p. Miller, R. C., 1972, Notes on analysis and severe storm forecasting procedures of the Air Force Global Weather Center: Air Weather Service Tech. Rept. 200 (revised), 102 p. National Oceanic and Atmospheric Administration, 1973, Monthly normals of temperature, precipitation, and heating and cooling degree days 1941—70, Climatography of the United States, No. 81, Colorado: US. Dept. of Commerce, Environmental Data Service. 1976, Climatological Data, Colorado, August 1976: vol. 81, no. 8. 82 FLOOD, JULY 31~AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO Snipes, R. J., and others, 1974, Floods of June 1965 in Arkansas River basin, Colorado, Kansas, and New Mexico: U.S. Geol. Survey Water-Supply Paper 1850—D, 97 p. U.S. Army Corps of Engineers, 1945, Storm rainfall in the United States: U.S. Army Corps Engineers report. 1976. Post-flood report, Big Thompson River, flood of 31 July—1 August 1976: Omaha, Nebr. U.S. Water Resources Council, 1976, A uniform technique for deter- mining flood flow frequencies: Washington, DC, Bulletin 17, 26 p., 14 apps. Williams, G., 1976, Application of the National Weather Service flash-flood program in the Western Region: Natl. Oceanic At- mospheric Adm. Tech. Mem. NWS WR—103, 20 p. A Acknowledgments .......................... Allenspark, rainfall record ................... Arapahoe County Airport ................... Arizona ................ Arkansas River valley .. .. Army Corps of Engineers .................... Asztalos, J ohn, thunderstorm photographs . . . Automobiles, flood damage .................. Bellvue .................................... flood frequency , . . flooding ........ rainfall record .......................... Rist Canyon ............................ Berthoud. discharge ........................ Big Gulch .................................. Big Thompson area. cloud height . . thunderstorms .............. Big Thompson Canyon discharge . . . Site 23 ................................. Big Thompson dam, flood damage ............ Big Thompson powerplant, flood damage ..... Big Thompson River ........................ cumulus clouds . . . damage ........ discharge ...... flood frequency ......................... gradients .............................. high-water marks . LaSalle ...... discharge ....... . . 100-year flood ...................... Site 1 .................................. Site 3 .................................. Site 6 .................................. Site 12 . . Site 22 . . source ................................ See also North Fork Big Thompson River. Big Thompson River basin, cloudburst phase . . rainfall record .......................... Big Thompson pipeline, flood damage ........ Big Thompson River tributary, Site 7 ......... Big Thompson River valley, sediment deposi- tion ............................ Birch ...................................... Black Creek, discharge ...................... Site 18 ................................. Board of County Commissioners. Larimer County, flood data ............... Boulder, thunderstorm ...................... Boxelder Creek, damage ..................... flooding minor .......................... Bucket-survey data ......................... Buckhorn Creek . . Site 26 ............... Buckhorn Reservoir Dam . Building moratorium ....................... Buildings, flood damage ..................... Bureau of Reclamation, hydrologic data ...... Lower Missouri Region, flood data ....... Page 46 28 10 25 43 70 49 49 70 76 66 65 77 INDEX C Page Cache 1a Poudre River ....................... 3, 67 cumulus clouds ......................... 21 damage ................................ 70 flood frequency ......................... 69 Fort Collins, discharge .................. 68 gradients .............. 5 Greeley ..... 68 Site 34 ...... . . , 80 Site 36 ................................. 80 Site 37 ................................. 81 source ................................. 5 See also North Fork Cache 1a Poudre River. Canada, southern ........................... 10 Casualties .......................... 3, 70 Cedar Cove .......................... 78 Centennial Sunday .......................... 1, 70 Chambers Lake Dam, failure ................. 80 Channel erosion ................... 57 Cirrus anvil .............. 43 Cloudburst phase . . 49 Clouds, bases .......................... 39, 43 cirrus anvil ........................ . . . 43 cloudburst phase ....................... 49 freezing level ........................... 43 height. Big Thompson area , . ........ 40 ice phase ..... .. ........ 43 maximum tops .. 43 stratocumulus .......................... 43 stratus ................................. 43 Cobb, C. Glenn ............................. 3 College Avenue. Fort Collins ................. 80 Colorado, eastern, thunderstorms . i 21 western, surface pressure ................ 13 Colorado Department of Natural Resources . . . 3, 71 Colorado Water Conservation Board ......... 3. 71, 72 Continental Divide ......................... . 21 Convection, free ............................ 10 Convective activity ......................... 21 Cottonwood ......... 5 Costs ................. 7O Cross-section data ..... 62 Cumulus clouds ............................ 21 D Dale Creek, flooding ........................ 67 Dale Creek tributary discharge ...... 67 Site 29 ......................... 79 Dam failures, Big Thompson darn ............ 70 Chambers Lake Dam .................... 80 Lily Lake Dam ...... 73 Olympus Dam ....... 62 Damage, property, total . . . . . 3, 70 Dark Gulch, discharge ...................... 63, 65 Site 8 .................................. 76 Data sources ............................... 71 Deadman Creek, discharge .................. 67 flood frequency ........ 69 Site 30 .......... 79 Deaths ........ 3, 70 Debris transport ............................ 57 Deep convection ............................ 21 Page Denver, rawinsonde data .................... 10, 40 thunderstorm ....... 32 trailing front ........ . . . 28 Devils Gulch, discharge ..................... 65 Site 16 ................................. 77 Dewpoint temperatures ..................... 12 Disaster area, official ....................... 70 Discharges, peak ...... 57 Diurnal heating 25 Douglas-fir ..... . . r 5 Drake ..................................... 3, 5 altitude ................................ 5 Big Thompson River .................... 78 cloudburst phase ....................... 49 discharge ......... 65 flood damage 70 flooding ..... i 56 Long Gulch ............................ 77 North Fork Big Thompson River ......... 77 North Fork Big Thompson River tribu- tary . 1 . ..................... 77 100-year flood . 69 Rabbit Gulch . . . . 76 thunderstorm, 1845 MDT . 43 Dry Gulch, peak discharges .................. 62 sediment deposition ..................... 74 Site 4 .................................. 75 75 E, F Environmental Research Laboratories ........ 32 Estes Park ................................. 3 cloudburst phase . .. 56 flooding .............. . 5, 56, 62. 69 Little Thompson River , 66, 79 Estes Powerplant ........................... 72 Evans Town ditch .......................... 79 Faults. stream control ...................... 4 Fish Creek, discharges . . 62 flooding ........ 62 Site 2 ..... . . 73 Flash flooding .............................. 5, 46 Glen Comfort ........................... 56 Flood areas ................................ 5, 62 Flood damage, Glen Haven .................. 65 West Creek ............. 65 Flood data ........ 56 Flood frequency . . . . 69 Flood profiles .............................. 62 Flood type ................................. 5 Flooding, comparisons ,. 71 greatest discharges 63 water velocity ...... 57 Foothills area .............................. 3 Foothills, thunderstorms .................... 39 Fort Collins ................................ 3, 28. 67 heavy rainfall ........................... 56 Cache la Poudre River . 80 rainfall record ........ 46 surface observation . 43 wind conditions ......................... 39 wind speed ............................. 32 83 84 Page Fox Creek. discharge ........................ 65 flooding ................................ 64 Site 14 ................................. 77 Freezing level . . . . 43 Frequency, flood . . 69 Front Range ..... .. 3 Fronts, leading ............................. 21 trailing ................................ 12, 21 G Gaging stations ...... . r 56 peak discharges ......................... 57 Geostationary Operational Environmental Satellite ........................ 11 Glen Comfort, cloudburst phase .............. 49 Dark Gulch ............................ 76 Noels Draw ................... 76 north bank tributaries, flooding . 63 rainfall rate ................... . . 56 thunderstorm cells ...................... 49 Glen Haven ................................ 5 Black Creek ............................ 77 cloudburst phase ....................... 56 cumulative rainfall . . 56 Devils Gulch ....... 77 discharge ...... . . 64 flood damage ........................... 65, 70 flood frequency ......................... 69 flood limit .............................. 5 Fox Creek .............................. 77 heavy rainfall . 64, 65 Miller Fork 1 r . 77 rainfall rate ........ 56 thunderstorm cells ...................... 49 West Creek ............................. 77 Glen Haven picnic ground, discharge ......... 65 GOES ..................................... 11 Gradients, streambed ............ 5 Grand Junction, rawinsonde data . . . 40 Great Lakes ..................... . . 10 Greeley .................................... 5, 28 Cache la Poudre River ................... 81 discharge ............ 68 wind speed ........................ 32 Green Ridge Glade, Big Thompson River . . . 78 discharge .............................. 66 Site 24 ................................. 78 Grover .................................... 28 radar .................................. 40, 43 rainfall total ........................ 56 H. I Hail ....................................... 9 Handy Ditch diversion dam ................. 78 Hanson Canal .............................. 78 High Plains .............. 10 central, deep convection . 21 High-water marks .......... . . 62 House trailers, flood damage ................. 70 Inversion .................................. 25 Isohyeta] map .............................. 43 J , K Jefferson County Airport .................... 28 Kansas .................................... 10 southern, thunderstorms ................ 21 L Lake Estes. dam gates ...................... 57 discharge .............................. 62 Laporte ..................... 68 Larimer County ............ . . 3 flood data .............................. 71 INDEX Page LaSalle .................................... 3, 5 Big Thompson River .................... 79 discharge .............................. 67 Leading front . . . 21 merger . . . . 25 Lewstone Creek . . 80 Lifted Index ............................... 10 Lily Lake. dam failure ....................... 73 Limon ....................... 28 radar .................... . 40, 43 rainfall total ........... . . . . 56 Lincoln Avenue Bridge, Fort Collins .......... 80 Little Thompson River ............. 3 discharge .............................. 66, 67 Estes Park ............................. 79 flooding ................ 66 Site 27 ................. 79 Livermore, flooding . 56, 67 heavy rainfall . . . ....... 56 Lone Pine Creek ........................ 80 North Fork Cache la Poudre River ........ 80 Stonewall Creek ........................ 79 Lone Pine Creek, discharge 67 Site 32 ............. 80 Long Gulch, discharge . . . . 64 Site 1 1 ................................. 77 Longmont, wind conditions .................. 39 Loveland, flooding .......................... 56. 66 wind conditions ......................... 39 wind speed ........... 32 Loveland Heights, damage ......... 70 north bank tributaries, flooding . . . 63 peak discharges ......................... 63 unnamed gulch, discharge ............... 63 Lyons, thunderstorm, 1800 MDT ............ 43 M Masonville, Redstone Creek ................. 66. 78 Meadowdale Ranch ......................... 79 Meteorology ............................... 9 Miller Fork, discharge ...... 65 Site 19 ................ 77 Mitchell Lake . . . . . . 43 Moisture content ........................... 10, 12 Montana ................................... 10 Mountain Avenue. Fort Collins .............. 80 N National Center for Atmospheric Research Field Observing Facility . . 3 National Climatic Center, flood data ..... . 71 National Hail Research Experiment site ...... 3, 10, 28 flood data .............................. 72 National Oceanic and Atmospheric Adminis- tration, flood data ............... 72 National Weather Service ..... 3, 71 Nebraska ....................... 10 surface pressure ....... 13 New Mexico ................................ 10 northern, thunderstorms ................ 21 Noels Draw, discharge ........ 63 high runoff ............. 66 sheet erosion 66 Site 9 ....... 76 North Fork Big Thompson River ............. 3 damage ................................ 70 discharge .............................. 65 flooding ................ 56 gradients .............. 5 high-water marks . . . 62 Site 13 ................................. 65, 77 Site 21 ................................. 65. 77 North Fork Big Thompson River tributary, Site 17 ......................... 77 Site 20 ...................... 77 North Fork Cache la Poudre River . . . . 3 flood crest .............................. 67 North Fork Cache la Poudre River—Continued Page flooding ................................ 56, 67 heavy rainfall ........................... 56 rainfall record .......................... 49 Site 33 ........ . 80 North Poudre Ditch ......................... 79 0, P Olympus Dam .............................. 64 erosion ................................. 62 flooding . . . . 62 gates closed . . . ...... . 62 Omaha District, US. Army Corps of Engineers flood data ...................... 72 Palmer Ridge. thunderstorms ................ 21 Parshall flume. buried ....................... 74 discharge rating . . . 73 Peak discharges ..... 57, 63 Peak stages ................. . 57 Photographs, thunderstorms ................ 43 Plants ..................................... 5 Polar air ..... 10 Ponderosa pine 1 . . . 5 Poudre Park ......... . . 5, 67. 68 Poudre Valley Canal ........................ 80 Precambrian rocks .......................... 3 Precipitable water .......................... 10 Precipitation ............................... 5 Precipitation summaries ..... 46 Preliminary streamflow data .. 62 Pressure ridge ............... 10 Pressure trough ............................ 11 Property damage, total ...................... 3 R Rabbit Gulch, discharge . . 64 Site 10 .............. 76 Radar, Grover .............................. 40 Limon ................................. 40 Radar data ................................ 28 Radar reflectivity, total rainfall equation . . . . 56 Radar summaries ..................... 10, 21 Rainfall, analysis . . . . 43 average ......... 5 floods .................................. 5 intensities .............................. 46 records . . i. 43, 49 total .......... 46 equation ........ 56 Rainstorms, comparisons ................... 71 Rawinsonde data ........................... 10 Denver ................................ 40 first ................................... 10 Grand Junction . . 40 second .. . . 12 Sterling ......... . r . 40 Table Mountain ........................ 40 third ................................... 21 Red Feather Lakes road, discharge ........... 67 Site 33 ................................. 80 Redstone Creek, discharge . . 66 Site 25 ............... 78 References, selected . . . . . 81 Reflectivity data ........................... 28 Rescue operations .......................... 70 Reservoir storage ........................... 67 Rist Canyon ................................ 3 discharge ....... 68 flood frequency . . 69 flooding ......... . . . 67 Site 35 ................................. 80 Rocks. Precambrian ........................ 3 Rockwell International Corp. 3 Rocky Flats. surface data . . 32 Rocky Flats plant ....... 3. 28 thunderstorm ..................... 38 Rocky Mountains, deep convection ........... 21 S Page Satellite photographs ....................... 10, 21 Search operations ......... . 70 Sheet erosion, Noels Draw . . . 66 Site 1, Lake Estes, discharge ................ 62 Big Thompson River .................... 72 Site 2, Fish Creek ........................... 73 flooding ................................ 62 Site 3, Big Thompson River . 74 Olympus Dam ......... 62 Site 4, Dry Gulch . . 62, 75 Site 5, Dry Gulch ........................... 62, 75 Site 6, Big Thompson River .................. 62, 75 Loveland Heights. discharge ..... r 63 Site 7, Big Thompson River tributary . . . . 63, 76 Site 8, Dark Gulch .................. . 63, 65, 76 Site 9, Noels Draw .......................... 63, 76 Site 10, Rabbit Gulch ....................... 64, 76 Site 11, Long Gulch ......................... 64, 77 Site 12, Big Thompson River ................ 64, 77 Drake, 100-year flood ............. . 69 Site 13, North Fork Big Thompson River . 65, 77 Site 14, Fox Creek .................... 65, 77 Site 15, West Creek ......................... 65, 77 Site 16, Devils Gulch ........................ 65, 77 Site 17, Glen Haven picnic ground, discharge . . 65 North Fork Big Thompson River tribu- tary ............................ 77 Site 18, Black Creek 65, 77 Site 19, Miller Fork ......................... 65, 77 Site 20, North Fork Big Thompson River tribu- tary ............................ 65, 77 Site 21, North Fork Big Thompson River ..... 65, 77 Site 22, Big Thompson River . . . .......... 78 Drake, discharge ........... 65 Site 23, Big Thompson Canyon ......... 66, 78 100-year flood .................... . 69 Site 24, Green Ridge Glade .................. 66, 78 Site 25, Redstone Creek ..................... 66, 78 Site 26, Buckhorn Creek ........ 66, 78 Site 27, Little Thompson River . . 66, 79 Site 28, Big Thompson River . 67, 79 Site 29, Dale Creek tributary ................. 67, 79 Site 30, Deadman Creek ..................... 67, 79 Site 31, Stonewall Creek ..................... 67, 79 Site 32, Lone Pine Creek ..................... 67, 80 Site 33, North Fork Cache la Poudre River . . 80 Red Feather Lakes road, discharge ....... 67 Site 34, Cache la Poudre River ......... . 68, 80 flood frequency ......................... 69 Site 35, Rist Canyon ........................ 68, 80 Site 36, Cache la Poudre River ............... 80 Fort Collins, discharge .................. 68 Site 37, Cache la Poudre River. . 81 Greeley, discharge ......... 68 6-month moratorium . . . . . 70 Slopes ..................................... 5 Snowmelt floods ............................ 5 Soils ............. 5 Soldier Canyon . . . Sources, data ............................... 71 INDEX Page South Platte River .......................... 3, 5 Southern Canada ..... . 10 Spring Canyon Dam ........................ 78 Squall line ................................. 21, 25 Stapleton International Airport ........ . 28, 32 State Highway 200, Red Feather Lakes . . . 80 Sterling, rawinsonde data ............. 10, 4O trailing front ................... 12 Stonewall Creek, runoff ............ . . 67 Site 31 ................................. 79 Storm development ......................... 10 0740 MDT ..... 10 1200 MDT . . . 12 1320 MDT . . . 12 1600 MDT 1730 MDT 1800 MDT 1830 MDT 1845 MDT 1900 MDT 1930 MDT ....... . 2000-0100 MDT ........................ 67 2130 MDT ............................. 62 2200 MDT ............................. 56 2315 MDT ............................. 68 0300 MDT, August 1 ......... 66 Black Creek, 2100—2300 MDT . . . 65 Drake, 2100 MDT ............ . . 65 Fox Creek, 2100 MDT ................... 64 Glen Haven, 1930 MDT ................. 64 Miller Creek, 2100—2300 MDT ........... 65 See also Thunderstorms. Storm Mountain, thunderstorm .............. 43 Stratocumulus cloud ........... 43 Stratus clouds .......... . . 43 Stream courses, faults ...................... 4 Stream depths ............................. 57 Stream velocities ........................... 57 Streambed gradients ........................ 5 Streamflow data, preliminary . 62 Surface low ................. 25 T 28 40 . . . 32 wind conditions ......................... 39 Temperature inversion ...................... 10 Terrain ............... 3 Texas .......... 10 Thermal structure .............. 1 1 Thunderstorm cells, Glen Haven ........ 49 Glen Comfort ..................... 49 Thunderstorms . . 1 cirrus anvil ...... 43 maximum tops .. . .. 43 movement .............................. 21 Palmer Ridge ........................... 21 photographs ........................... 43 85 Thunderstorms—Continued Page steady state ............................ 43 stream velocities ........................ 57 Total rainfall ............................... 46 Totals Index ............................... 10 Trailing front ....................... 12, 21, 25, 28, 32 foothills ..................... 39 maximum intensity .. . 25 merger ............. . 25 stationary .............................. 38 thunderstorms ......................... 21 Trees ...................................... 5 True Gulch, discharge ....................... 64 U, V University of Northern Colorado ............. 3 US Army Corps of Engineers ............... 3, 72 US Bureau of Reclamation ................. 3 US Geological Survey, Water Resources Divi- sion, flood data .................. 71 US Highway 34, Dry Gulch... 62, 75 flood damage costs ...... . . . 70 Green Ridge Glade ...................... 78 US. Highway 287 .......................... 80 bank erosion . . . ......... 67 flooding ................ 66 Virginia Dale . . . . . 79 Utah .................... . r . 10 southern, squall line ..................... 21 Valley water storage ........................ 67 Video Integrator Processor , . ....... 32, 43 Virginia Dale .................... 3 Dale Creek tributary . . 79 Deadman Creek ............... 79 flood frequency ............. . . . 69 flooding ................................ 56, 67 heavy rainfall ........................... 56 rainfall record .......................... 49 W Waltonia, east, little flooding . 63 west, peak discharges ........... 63 Water Conservation Board, Colorado . . . 3 Wave Propagation Laboratory ......... . . 32 Weak trough ............................... 10 Weather fronts ............................. 10 Weld County ..... 3 road damage .. .. 70 West Creek, discharge . 65 flood damage ....................... 65 Site 15 ............................. . . 77 West, John M ............................... 3 Willow ..................................... 5 Wind shear . . . 43 Wind shift 38 Wind speeds , . 10, 32 Winds ..................................... 10, 25 Part B. Geologic and Geomorphic Effects in the Big Thompson Canyon Area, Larimer County By RALPH R. SHROBA, PAUL W. SCHMIDT, ELEANOR J. CROSBY, and WALLACE R. HANSEN of the US. GEOLOGICAL SURVEY With a section on DAMAGE CAUSED BY GEOLOGICAL PROCESSES DURING FLOOD PRODUCING STORMS By JAMES M. SOULE of the COLORADO GEOLOGICAL SURVEY STORM AND FLOOD OF JULY 31—AUGUST 1, 1976, IN THE BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS, LARIMER AND WELD COUNTIES, COLORADO GEOLOGICAL SURVEY PROFESSIONAL PAPER 1115 CONTENTS Page Page Abstract ................................................ 87 Geologic and geomorphic effects of the flood—Continued Introduction ............................................ 87 North Fork Big Thompson River ...................... 118 Grain sizes of sediments .............................. 88 Above Glen Haven .............................. 118 Converting English units to metric .................... 88 Devils Gulch ................................ 119 Acknowledgments ................................... 88 West Creek ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, 119 Physical setting of the Big Thompson River basin and its West of Glen Haven ......................... 120 relation to the flood ................................ 89 Glen Haven t0 mile 67, including Piper Meadows Tapography and drainage ---------------------------- 89 drainage .................................. 12o Geologic factors related to the flood ................... 91 Mile 6.7 to mile 50 ............................... 123 Bedrock ........................................ 91 Dunraven Glade ............................. 123 Surficial deposits -------------------------------- 92 Miller Fork ................................. 124 Vegetation .......................................... 94 Other tributaries ,,,,,,,,,,,,,,,,,,,,,,,,,,,, 124 Plains grassland ................................. 94 Debris avalanches ,,,,,,,,,,,,,,,,,,,,,,,,,,, 124 Montane forest .................................. 94 Mile 5.0 to Drake ................................ 124 Geomorphic evidence of past flooding ..................... 95 Big Thompson River from canyon mouth to conflu- Geologic and geomorphic effects of the flood ............... 95 ence with South Platte River '''''''''''''''''' 126 Big Thompson River and tributaries from Estes Park to Channel modification ............................ 126 Drake ............................................ 95 Overbank deposition ............................ 129 Estes Park area ................................. 95 Zone 1‘ Canyon mouth to Big Thompson school 129 Valley slopes ................................ 95 Zone 2, East of Big Thompson School to Love- Dry Gulch ---------------------------------- 100 land ...................................... 133 Big Thompson .Canyon, Olympus Dam to Drake . . . . 100 Zone 3' Southeast of Loveland to Larimer- Sheet eroswn and gullylng """""""""" 100 Weld County-line road ..................... 135 . Landshdmg ................................. 101 Zone 4’ Larimer-Weld County-line road to com Ma]or tributary gulches .......................... 103 fluence with South Platte River ......... 136 Minor tributary gulches .......................... 106 Damage caused by geologic processes during flood-producing Big Thompson River ............................. 106 . storms, by James M. Soule ......................... 136 Olympus Dam to Loveland Heights ........... 106 . . Geologlc hazards .................................... 136 Loveland Heights to Drake ................... 107 . . . . Damage in geologlc hazard areas ...................... 138 Big Thompson River from Drake to the canyon mouth . . 109 . . Damage on debris fans ........................... 138 Drake to Midway ................................ 110 . . . . Damage on valley slopes and in tributary dramages 139 Midway to Cedar Cove ........................... 110 Dama e in landslide areas 139 The Covered Wagon Restaurant area .......... 113 _ g """""""""""" Loveland powerplant and vicinity ,,,,,,,,,,,,, 114 Conclusrons ............................................. 140 Cedar Cove area ............................. 115 Selected references ...................................... 146 The Narrows ................................ 117 Index ................................................... 149 ILLUSTRATIONS Page PLATE 1. Map showing geomorphic setting of the Big Thompson drainage basin, north-central Colorado ............. In pocket 2. Map showing geologic and geomorphic effects of 1976 storm and flood, Big Thompson River area, Colorado . . In pocket 3. Stream profiles of Big Thompson River and North Fork Big Thompson River ............................. In pocket FIGURE 62. Map showing geomorphic setting of the Big Thompson River drainage basin, north-central Colorado ........... 90 63. Photograph of The Narrows of Big Thompson Canyon, August 2, 1976 ....................................... 93 64. Schematic diagram showing three modes of flood erosion and deposition within the canyons of the Big Thompson River and North Fork Big Thompson River ............................................................ 96 III IV CONTENTS FIGURE 65—99. Photographs: 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 85. 86. 87. 88. 89. 90. 91. 92. 93. 94. 95. 96. 97. 98. 99. Helicopter view showing effects of sheetflooding along foot of Mount Olympus at Crocker Ranch ...... Tongue of bouldery debris deposited by sheetflooding along the base of Mount Olympus at Crocker Ranch ..................................................................................... Toe of Olympus Dam, scoured by floodwaters of Dry Gulch, viewed toward the south ................ Small debris slide in stony colluvium on 67-percent slope near mile 53.2 ............................. Motel unit damaged by a rock slide on an oversteepened bedrock slope near mile 56.0 ................. Unnamed gulch, viewed about 100 yd upstream from its confluence with the Big Thompson River, at mile 54.4 ................................................................................... Gully cut by intense runoff channeled along the shallow ruts of a dirt road in a grassy area, upper part of unnamed gulch at mile 54.4 ............................................................... Granite boulder near mile 42.7, showing fresh impact scars ......................................... Overbank sand and buoyant debris on the south side of the Big Thompson River at mile 54.4 .......... Debris pile on the upstream side of highway bridge at mile 52.2 ..................................... House undercut by Big Thompson River near mile 45.4 ............................................ The riverside community of Waltonia, mile 47.0, before and after the 1976 flood ...................... Helicopter view showing flood damage in part of Drake ............................................ Preflood and postflood views taken near south end of Drake ........................................ Wrecked house and other debris lodged on a damaged private bridge at mile 44.7, 0.5 mi downstream from Drake ................................................................................ Flood damage along the Big Thompson River near center of Drake .................................. Remains of diversion dam below Midway at mile 43.25 ............................................. Site of the Loveland powerplant at mile 41.3 before and after the flood .............................. Helicopter view of the Cedar Cove area near mile 39.6 .............................................. Flood damage at the mouth of Big Thompson Canyon, U.S. Highway 34 ............................ View into The Narrows of Big Thompson Canyon on August 2, 197 6 ................................ House in the lower end of Devils Gulch, near Glen Haven, demolished by hydraulic forces and impact from floating debris ........................................................................ Scar left by debris avalanche that entered the North Fork at mile 5.1 ................................ Remains of a medium-size automobile flattened and wrapped around a large boulder in the North Fork near mile 4.5 ............................................................................... Pickup truck partly buried by coarse sand deposited by the North Fork near its confluence with the Big Thompson ................................................................................. View of damage to road and abutment of steel bridge torn out by Big Thompson River in crossing first ridge east of Big Thompson Canyon .......................................................... Lateral cut into south bank of the Big Thompson River southwest of Loveland ...................... Movement of cobbles and pebbles on a point bar indicated by partial burial of cattails ................ Flood-deposited sand near confluence of Sulzer Gulch and Big Thompson River showing two terraces cut below highest level of deposition ......................................................... Desiccation cracks in slightly silty very fine sand trapped at mouth of Sulzer Gulch .................. Upstream side of ponderosa pines stripped of bark by impact of flood debris, between dam at mouth of Big Thompson Canyon and first ridge east of canyon .......................................... Current-terraced deposit of sand on downstream side of debris pile in cottonwood grove west of Big Thompson School .......................................................................... Condemned damaged house and debris pile in cottonwood grove west of Big Thompson School ........ Lee deposits of flood-borne sand on down-current side of bushy weeds east of Glade Road ............. Flood damage in cornfield southwest of Loveland ................................................. 100. Map of geologic hazards in the Glen Comfort area, between mile 53.4 and mile 51.9 on the Big Thompson River . . 101—109. Photographs: 101. 102. 103. 104. 105. 106. 107. 108. 109. Aerial views of Noels Draw, mile 52.7 on the Big Thompson River, before and after the 1976 flood ..... Aerial views of debris fans, mile 51.1 to mile 50.9 on the Big Thompson River, before and after 1976 flood ...................................................................................... House on debris fan at mile 51.1, Big Thompson River, damaged by boulder debris ................... Deposit of sand- to cobble-size material on a debris fan near mile 7.3, North Fork Big Thompson River . Postflood aerial view of Bobcat Gulch debris fan, mile 0.5 on the North Fork Big Thompson River, August 3, 1976 ............................................................................. Building lot covered by sediment, in a subdivision where colluvium derived from grus was eroded and redeposited ................................................................................ Gully erosion in fine-grained, relatively thick colluvium or residuum derived from granitic rocks ....... Landslide in Glen Comfort area. mile 52.4 on the Big Thompson River ............................... Aerial view of debris avalanches from 1976 storm and flood. between mile 5.2 and mile 5.4 on the North Fork Big Thompson River ................................................................... Page 100 101 102 103 104 104 105 107 108 108 110 111 112 114 116 116 117 118 120 121 122 123 125 126 127 128 128 129 130 131 132 132 133 134 135 137 140 142 143 143 144 145 146 147 148 CONTENTS v TABLES Page TABLE 6. Summary of geologic and geomorphic effects of the July 31—August 1 storm and flood on the Big Thompson River and its tributaries from Olympus Dam to the confluence with the South Platte River, including average gradients and selected hydrologic data ........................................................................ 98 7. Size and lithology of largest boulders transported by the 1976 flood in the Big Thompson Canyon, mile 56.4 to mile 45.7 .................................................................................................... 106 8. Size and lithology of largest boulders deposited by the 1976 flood in the Big Thompson Canyon, mile 44.3 to mile 39.5 ........................................................................................................ 113 9. Concrete blocks carried downstream from destroyed diversion dam at river mile 43.25 ................................. 113 STORM AND FLOOD OF JULY PSI—AUGUST 1, 1976, IN THE BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS, LARIMER AND WELD COUNTIES, COLORADO GEOLOGIC AND GEOMORPHIC EFFECTS IN THE BIG THOMPSON CANYON AREA, LARIMER COUNTY By RALPH R. SHROBA, PAUL W. SCHMIDT, ELEANOR J. CROSBY, and WALLACE R. HANSEN, of the US. GEOLOGICAL SURVEY ABSTRACT Intense rainfall from the Big Thompson thunderstorm complex on the evening of July 31, 197 6, and the ensuing floods that evening and the following day led to widespread erosive and depositional ef- fects along the Big Thompson River and its tributaries. Because of the intensity of the rainfall and the steepness of the terrain, runoff quickly reached flood stage. The storm was centered over rugged country. Mountain tops are as much as 3,000 ft (feet) above stream level, and the canyon walls are commonly as steep as 80 percent (40°). Cliffs are common. The canyon is cut mostly into Precambrian metamorphic and igneous rocks, and stream gradients are closely related to the erosive resistance of the rocks. The steepest reaches are across resistant pegmatite; the flattest are across broad fault zones. Fault zones, in places, also control the trend of drainage. On the Great Plains the Big Thompson River crosses rocks of Paleozoic and Mesozoic age mantled with Quaternary deposits, and the gradient flattens ac- cordingly. In general, the river is at grade on the plains but not in the mountains. The flood responded markedly to changes in gradient. Reaches steeper than 2 percent generally were scoured, especially on the out- sides of bends and where the channel was constricted. Basically, the main channel was scoured throughout its length, but outside the main channel deposition took place where the gradient flattened to less than 2 percent, especially at wide places on the flood plain and on the insides of bends, where point bars formed. Large boulder- gravel bars diverted the river from its preflood channel, notably at the community of Drake, and deposition was intensified where the flow was impeded by bridges, buildings, or vegetation. Boulders as large as 7 ft in intermediate diameter were deposited in bars. The largest boulder apparently moved by the flood measured about 12>< 12x23 ft and weighed an estimated 275 tons. Lateral scour was most pronounced at the outsides of bends; in general, highway damage was greatest at such places. At The Narrows at the mouth of Big Thompson Canyon, scouring flood- water rose 14 ft above the preflood level, and obliterated 1.9 miles of highway. Downstream from the mouth of the canyon, overbank deposition was the chief geologic effect of the flood, although scour was ap- preciable at the outsides of bends and in the main channel. Over- bank deposits of sand and silt decreased in thickness and grain size from the canyon mouth to the South Platte River. Local deposits of gravel and cobbles were reworked from the preflood riverbed and banks. Buoyant debris was abundant. Croplands were damaged by deposits of silt and sand. Along the tributaries, geologic effects were confined largely to the zone of intense downpour, chiefly to places within the 6-in. (inch) isohyet and, foremost, within the 10-in. isohyet. Sheetflooding on hillslopes locally removed as much as a foot of soil and left pebbly to bouldery lag deposits. Locally, boulders were transported down slopes by sheetflooding and were redeposited on gentler slopes. Thickly grassed open slopes resisted sheet erosion rather well, but gullying quickly cut through the soil where the grass had been worn thin along dirt roads and trails. Sparsely covered slopes in pine forests were vulnerable to sheetflooding. Along minor drainageways, channelized runoff commonly scoured down to bedrock, then deposited debris flows or debris fans where reduced gradients lowered the competence of the floodwater. Some major tributaries dumped their loads into the main stem where the load was either deposited as an obstructing bar or was carried off by the flood. In places the Big Thompson River and the North Fork were forced laterally out of their preflood channels by swollen tributaries. Debris slides, debris flows, debris avalanches, rock slides, and rock falls were set off in the intense-downpour area. Saturation of . the soil was the chief cause, but some landslides were caused by lateral undercutting. Many homes and other buildings were lifted off their foundations and were carried away by the flood. Some of these collapsed with nearly explosive force. Some buildings that remained in place were partly filled with sediment and were damaged by impact from floating and saltating debris. Massive piles of buoyant debris ac- cumulated against trees and buildings. Trees, bushes, and even grasses acted as natural sediment traps. Historic, climatologic, hydrologic, and geologic evidence indicates that other drainage basins along the east slope of the Front Range are no less vulnerable than the Big Thompson River basin to catastrophic flooding in the future. INTRODUCTION Mountain flooding is not uncommon in the Front Range of Colorado, and previous floods have also pro- duced marked geologic and geomorphic changes. No previous historic Front Range flood, however, has in- volved so much erosion and material transport, and never before has a Front Range storm of such magnitude afforded such an opportunity for study and 87 88 FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO documentation. Centered over Big Thompson Canyon and its North Fork between Estes Park and Drake (pl. 1), uncommonly heavy rains falling on steep slopes led to very rapid runoff, high flow velocities and flood peaks, widespread geologic and geomorphic changes, heavy property damages, and high mortality among canyon residents and visitors. Maximum rainfall inten- sity was about 10 in. in less than 4 hours (hrs). An estimated 6 in. fell at Glen Haven in about 45 minutes (min), and a total of 12 in. fell in 48 hrs. (See Part A, fig. 44.) Hydrologic measurements at the canyon mouth in- dicate a recurrence interval of about 200 yrs. Small tributary basins had discharges unequalled in the records of Colorado flood history, and flow velocities reached about 10-30 ft/s (Part A, table 4). With such intensities of rainfall and runoff, widespread geologic- geomorphic effects were inevitable. Coming on a weekend during the peak of the summer vacation season—with an estimated 2,500 people in the canyon—the Big Thompson flood caught hundreds of residents, campers, and tourists with virtually no warning. It left 139 known dead and, at this writing, 5 persons unaccounted for. Although a detailed discussion of the socioeconomic impact of the Big Thompson storm is outside the scope of this report, it was of great concern to many people, and we would be remiss not to note briefly some of the costs. The economic losses were severe, amounting to an estimated $355+ million.1 Tables prepared by the U.S. Army Corps of Engineers, Omaha District (James W. Ray, letter dated Dec. 30, 1976), list financial losses caused by the Big Thompson storm in various categories. Most of the damage in the public sector was to highways, roads, and bridges; whereas most of the private damage befell individuals and small businesses. A damage survey by Larimer County in- dicated that 252 structures, mostly dwelling units, were damaged beyond repair (damaged more than 50 percent—condemned and demolished); 242 structures were damaged to the extent of less than 50 percent and, therefore, were declared eligible for repair. The U.S. Army Corps of Engineers awarded contracts for the removal of 319,863 yd3 of debris, 93 propane tanks, and 197 automobiles. Debris included flotsam as well as structures that were damaged beyond salvage and had to be demolished. ‘Even 50. property damages from the Big Thompson flood were far less than from some previous floods. The storm of June 14—17. 1965, over the South Platte River basin cost an estimated $508.2 million in damages, about 75 percent of which occurred in the Greater Denver Area (Matthai, 1969, p. B31). The storm of May 5—6, 1973, in the Greater Denver Area (Hansen, 1973; Ducret and Hansen, 1973) cost an estimated $50 million. GRAIN SIZES OF SEDIMENTS Grain sizes of sand, gravel, and so forth noted in this report are based mostly on visual estimates or field measurements rather than on laboratory measurements. Because of the extreme heterogeneity of most of the flood deposits, precise laboratory measurements of small samples, in fact, could yield misleading results. Our field nomenclature for these materials is modified from the widely used United Soil Classification System adopted by the U.S. Bureau of Reclamation and the U.S. Army Corps of Engineers as follows: SIEVE NO. (OPENINGS PER INCH) INCHES 200 40 10 4 3 12 clay and fine medium coarse pebble cobble boulder Silt sand sand sand gravel gravel gravel CONVERTING ENGLISH UNITS TO METRIC With the passage by Congress of the Metric Conver- sion Act of 1975, agencies of the United States Government, nearly all scientific and research organizations, and many private industrial organiza- tions are rapidly converting measurements from the Customary System of Units (English) to the Interna- tional System of Units (metric). The Geological Survey has been in the forefront, but inasmuch as many readers of this report probably are not yet comfortable with the International System, customary units are used here. ACKNOWLEDGMENTS This report is based on information and assistance from many sources. Local residents and other private individuals, and county, State, and Federal agencies all contributed data and cooperation in the field. We thank the many landowners who permitted access to or through their properties. Sheriff Bob Watson and his staff in Larimer County extended their cooperation, eased access to the devastated areas, and provided much basic information. Helicopter support was pro- vided by the Colorado National Guard, the U.S. Air Force, and private firms. The Colorado Geological Survey cooperated in many ways, both in the field and in the office. Larry W. Anderson, H. Kit Fuller, William Markovic, and Roderic A. Parnell, Jr., assisted in the field on numerous occasions. GEOLOGIC AND GEOMORPHIC EFFECTS, BIG THOMPSON CANYON AREA 89 PHYSICAL SETTING OF THE BIG THOMPSON RIVER BASIN AND ITS RELATION TO THE FLOOD TOPOGRAPHY AND DRAINAGE The Big Thompson River arises near timberline just below the Continental Divide at an altitude of about 11,000 ft in Rocky Mountain National Park, north- central Colorado (fig. 62). Some of its tributaries arise even higher from small glaciers. The headwaters are rimmed on the west, north, and south by many peaks that stand more than 12,000 ft above mean sea level, including Longs Peak, which at 14,255 ft is the highest summit in the watershed. The Big Thompson River descends about 6,330 ft in 55 mi—as a crow flies—from its sources to its confluence with the South Platte River on the Great Plains at an altitude of about 4,670 ft. The Big Thompson River descends about 3,500 ft in 17 mi as it flows eastward from its source to the town of Estes Park at the west edge of the downpour area of the Big Thompson River flood. Estes Park is about 7,500 ft above sea level and is about 50 mi northwest of Denver (pl. 1). Estes Park is named for a broad grassy mountain valley (or “park”) of the same name sur- rounded by crags and forested mountains. Just downstream from Estes Park the Big Thompson River enters the narrow Big Thompson Canyon. Most of the severe flooding took place in the canyon and its tributary gulches. The east edge of Estes Park and the head of Big Thompson Canyon about coincide with the west edge of the heavy rainstorm that caused the July 31—August 1, 1976, Big Thompson flood (pl. 2A). Three permanent streams, Fish Creek from the south and Fall River and Black Canyon Creek from the northwest, join the Big Thompson just above Estes Lake, which is impounded by Olympus Dam at the east edge of the park. These three streams were not in- volved in the flood. Dry Gulch, which was heavily in- volved in the flood, drains into the Big Thompson from the north just below the dam. Olympus Dam is about 1% mi downstream from Estes Park. The dam serves chiefly as a power struc- ture, and its modest reservoir capacity (Lake Estes) was not designed for flood control, especially since the water level must be kept high for power production. Nevertheless, the outlet from the reservoir to the Big Thompson River was closed by the US. Bureau of Reclamation at 2055 MDT after the first reports of flooding, and the inflow from the river to the reser- voir—about 400 ftS/s—was diverted to the Olympus tunnel, which bypasses the canyon. Ultimately this in- flow was stored east of the mountains at Carter Lake reservoir. The entire inflow of the Big Thompson River above the dam was so diverted during salvage and cleanup operations. Just east of Lake Estes at Olympus Heights, the river enters Big Thompson Canyon. It emerges from the canyon 20 mi downstream at the mountain front. The North Fork Big Thompson River, the largest tributary within the canyon, arises among the high peaks of the Mummy Range to the northwest (also within Rocky Mountain National Park) and joins the main stem at Drake, about two-thirds of the distance down the canyon. Many smaller gulches and draws join the Big Thompson River and the North Fork in the canyon section, and many of them released excep- tional discharges of water during the 1976 flood. The storm of July 31, 1976, centered over rugged country, and the steepness of the terrain helped pro- mote the rapid runoff that led to the catastrophic flooding. Mountain tops are as much as 2,000—3,000 ft above stream level, and the steep canyon walls com- monly have slopes of 40—80 percent (20°—40°); locally, the slopes are much steeper. Slopes of 10 percent or less (about 5°) are exceptional. Along much of Big Thompson Canyon the canyon walls rise directly from the water’s edge. Along discontinuous flood plains and at the mouths of large tributaries, the canyon floor is as much as 500 ft wide in some places, but it general- ly is considerably less. US. Highway 34 through Big Thompson Canyon utilized flood-plain surfaces and low terraces where they provided a practical base, but much of the road was constructed on manmade em- bankments built against the canyon walls. Since the flood the road has been rebuilt in much the same fashion. From Olympus Dam to the canyon mouth, the Big Thompson River descends more than 2,100 ft in 20 mi (pl. 4). The average gradient in this distance is about 2 percent, or a little more than 100 ft/mi. In the upper third of the canyon, above Waltonia, the gradient is about 1 percent. Downstream from Waltonia it varies locally from as much as 7 percent, to as little as 0.75 percent. On the North Fork, from Devils Gulch near Glen Haven to the main river at Drake, the channel gradient averages about 2 percent. Major tributary gulches that drained the heavy precipitation of July 31, 1976, have higher gradients: 8 percent in Noels Draw, 10 percent in Long Gulch, and 12.5 percent in Dark Gulch. Smaller, shorter gulches and gullies have even steeper gradients. From Drake downstream to the canyon mouth, Big Thompson Canyon varies from a wide valley localized by faulting to a steep-sided narrow canyon cut into metamorphic rocks. Along this 8-mi stretch, the river drops more than 800 ft, and the gradient ranges from mo “105°00' 105°45' mHOOU. 5‘94 wHI>CQCwe H. 5.5. wHQ Hmogwmoz 533w? OOEON>UO ‘ Basin boundary north-central Colorado. FIGURE 62.—Map showing geomorphic setting of the Big Thompson River drainage basin, GEOLOGIC AND GEOMORPHIC EFFECTS, BIG THOMPSON CANYON AREA 91 less than 1 percent to more than 4 percent. The width of the canyon floor ranges from less than 100 ft to as much as 600 ft. Wide places in the canyon have been much used as building sites. Ease of access is com- bined with favorable soil conditions for excavating foundations and for installing wells and waste disposal systems. Drake, Midway, and Cedar Cove became focal points for summer and year-around homes and for tourist-oriented businesses, such as motels, restaurants, trailer courts, and shops. About half a mile below Midway, the river had been dammed for a downstream hydroelectric powerplant serving the town of Loveland. Below Cedar Cove some water had been diverted for cropland irrigation. East of the mountain front, the Big Thompson River flows generally eastward about 35 mi across the Col- orado Piedmont section of the Great Plains to its con- fluence with the South Platte River (pl. 1). Between the mountain front and Loveland, the river crosses a belt of alternate strike valleys and hogback ridges, formed by differential erosion of interlayered soft and hard, tilted strata. This belt, however, has little effect on the behavior of the river; the river crosses it with no significant change of gradient. From the Loveland area to the South Platte, the river flows between discon- tinuous alluvial terraces on a flood plain that locally is more than a mile wide. The river gradient is about 0.7 percent from the canyon mouth to Loveland and about 0.2 percent (or about 10.6 ft/mi) from Loveland to the South Platte River. GEOLOGIC FACTORS RELATED TO THE FLOOD BEDROCK Bedrock at first thought might seem to have little in- fluence on the behavior of the 1976 flood or its geomor- phic effects. Closer inspection indicates clear cause- and-effect relationships—some subtle and some more obvious. Canyon widths are related to the erosive resistance of the bedrock and local base levels; the gra- dient is related both to the resistance of the bedrock and to the width of the canyon; the velocity and com- petence of the river in flood, in turn, are closely related to the gradient. Between its source and its confluence with the South Platte River, the Big Thompson River flows across rocks of widely varied composition and age. The moun- tainward part of the basin is eroded entirely from crystalline rocks of Precambrian age; the greater part of the canyon, along both the main stem and the North Fork, is cut into a metamorphic complex of gneiss, schist, and migmatite (Tweto, 1976). This complex is intruded by many small stocks of Boulder Creek Granodiorite and is bordered irregularly on the west by a batholith of Silver Plume Granite. In addition, the metamorphic complex is intruded by uncounted pegmatite dikes and lesser plutonic bodies. Pegmatite is especially abundant in the area between Long Gulch, Glen Haven, and Drake. At the mouth of Big Thompson Canyon the river passes from the Precambrian terrane onto a belt of tilted sedimentary rocks of successively younger Paleozoic and Mesozoic age. This terrane is com- plicated somewhat by faulting and folding, which cause stratigraphic repetitions across the strike and even bring Precambrian rocks to the surface locally east of the mountain front. From a point just west of Loveland to the South Platte River, the Big Thompson River flows across a thick sequence of gently dipping Upper Cretaceous rocks which, however, are beveled almost flat and are largely concealed by surficial deposits of Quaternary age (Colton, 1978). The bedrock geology of the Big Thompson area from the hogbacks west of Loveland into the mountains is complicated by abundant faults and shear zones of varied ages, sizes, and orientations. The predominant trend of fractures is northwestward, and many dikes follow that trend also. From Drake east toward the mountain front and down the North Fork from Dunraven Glade, the Big Thompson River and the North Fork cross and recross a broad west-northwest- trending fault zone called the Thompson Canyon fault (Braddock, Calvert, and others, 1970). This zone has been mapped for a distance of about 24 mi (Tweto, 1976). In many places the fault zone and the canyon floor coincide, so the fault zone must have influenced the course of the river. At Cedar Cove the zone probably exceeds 600 ft in width (Braddock, Nutalaya, and others, 1970). Toward the canyon mouth the river swings north away from the zone and passes instead through The Narrows in resistant nearly vertical metasedimentary layers (Braddock, Calvert, and others, 1970). At the west end of Big Thompson Canyon, a north- northeast-trending fault about 12 mi long borders Estes Park on the east. Dry Gulch, Devils Gulch, and Fish Creek about coincide with its trace. Many other gulches and tributary ravines also coincide with faults in the Big Thompson area and must have been localiz- ed by them, through the process of differential erosion. Between Olympus Dam and river mile 50.5 (miles in- dicated on pl. 2A, 23, 20), the river crosses a large body of granitic rock, mostly of the Silver Plume vari- ety, and the gradient is about 60 ft/mi (pl. 3). At about river mile 48, the river crosses onto a terrane of very resistant pegmatite interlayered with metamorphic rocks. Here, the gradient steepens to about 160 ft/mi; the canyon is constricted and steep sided, and the bed 92 FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO of the river is very bouldery. These conditions persist downstream about to Drake. At and below Drake, and up the North Fork from Drake, the valley bottom coin- cides with the Thompson Canyon fault, the canyon widens conspicuously, and the gradient flattens. Where the river swings away from the fault, the canyon again constricts and steepens just as noticeably—for example, below Midway, above Cedar Cove, and at The Narrows. In the wider reaches, the river down through time has built discontinuous flood plains, commonly as much as 500 ft across and locally much wider. The July 31 flood spread out in these bottomlands and deposited much of its load. At Drake the river deposited a huge bar of cobbles and boulders across the old preflood channel; in slacker water it deposited sand and pebble gravel along with a great mismash of tree trunks, branches, lumber, fuel tanks, household items, and parts of demolished buildings. Conversely, scour predominated in the steeper, narrower reaches; for example, 1.9 mi of highway was destroyed at The Narrows when the highway embankment was obliterated (fig. 63). These details are further discussed in subsequent sections. Subtle effects of bedrock on the degradation and ag- gradation patterns of the flood extend along the main drainages and up the tributaries. Granitic terrane in Big Thompson Canyon generally has yielded larger and more abundant boulders to erosive processes than metamorphic terrane has, partly because the granite is more massive, and the metamorphic rock tends to be strongly foliated and closely jointed. Consequently, reaches of the river flowing across granite tend to be rougher and more bouldery than reaches flowing across metamorphic rocks. SURFICIAL DEPOSITS Surficial deposits greatly influenced the distribution and character of the effects of the flood. To a large extent they determined the sediment yield of the involved streams and, hence, the erosion and sedimen- tation effects on the whole drainage system. Surficial deposits mantle bedrock discontinuously throughout the study area. In most places, except in valley bottoms, the surficial mantle is patchy and thin; in many places, bedrock forms cliffs or bold outcrops. In its headwaters the Big Thompson River crosses deposits laid down by a succession of late Pleistocene glaciers. These glaciers terminated just west of the town of Estes Park where they built massive moraines, but outwash gravel extended on down the canyon and out onto the plains. Down through the years, much of the outwash has been reworked by the Big Thompson River, but patches still remain in the canyon. In many places in the canyon, hillsides are mantled with soil or with stony sandy colluvium and slope wash to depths of several feet. The colluvium occasionally passes upslope into coarse talus and downslope into alluvium. Loamy soils high in some tributaries, such as Piper Meadows, may have been augmented by eolian deposition. During the July 31, 1976 storm, much saturated colluvium collapsed and moved downslope as small landslides. Also, much colluvium was transported down hillsides and into the drainage courses by sheetflooding, rill erosion, and gullying in countless small channels. On gently sloping granitic terrane, especially on Silver Plume Granite, prolonged weathering has pro- duced a patchy friable grus and (or) sandy colluvium, commonly 3 ft or more deep, interspersed with round- ed outcrops. This kind of regolith is very vulnerable to scour, and it was severely eroded locally on July 31, 1976. Regoliths on metamorphic terranes seemed to have been less vulnerable to scour, but the reasons why are unclear; factors such as rainfall intensity and discharge might have been as important as erodibility. The floors of most sizeable tributary canyons, gulches, and ravines are bottomed with nearly con- tinuous fills of alluvium and slope wash. Many of these were severely scoured during the storm. Most of them have soil profiles that indicate ages of at least several hundreds of years. The Big Thompson itself flows across a virtually continuous fill of mostly coarse grained bouldery alluvium, which in wide places such as Drake covers several acres. The course of the river, moreover, is bordered by discontinuous gravel ter- races, some of which were drastically altered by the flood. Some buildings on terraces or colluvial SIOpes above the flood were carried away when lateral scour caused the banks to slump. During the flood, many alluviated tributaries were scoured to bedrock, especially toward their mouths, through old fills as much as 10 ft thick (fig. 70). Although in many places the Big Thompson severely scoured its bed, it rarely uncovered bedrock in the bot- tom of its channel; but in many places it cut laterally into alluvial terraces, colluvial slopes, and road em- bankments; and it released many small landslides. As it scoured the channel floor and reworked and redeposited its bedload, its channel shifted from side to side, causing much property damage in the process. Nearly all large tributaries of the Big Thompson and the North Fork, and many small tributaries, have debris fans at their mouths. (See “Glossary,” fig. B.) These fans are the products of multiple episodes of deposition, and most of them are graded to the flood plain or to slightly higher stands of the river. Nearly all of them in the area of heavy downpour were modified by the storm. They were variously entrench- GEOLOGIC AND GEOMORPHIC EFFECTS, BIG THOMPSON CANYON AREA FIGURE 63.—The Narrows of Big Thompson Canyon, August 2, 1976, after the flood had receded. US. Highway 34, obliterated in this section of canyon, had followed the left bank in this View looking upstream. Valley floor here is about 80—100 ft wide. 94 FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO ed or aggraded by runoff down their own drainages, or they were truncated by the Big Thompson; some fans deflected the course of the Big Thompson. Fans were affected by combinations of four processes: (1) en- trenchment, (2) aggradation, (3) truncation by the Big Thompson or the North Fork, and (4) deflection of the Big Thompson or the North Fork. All four processes caused extensive damage. As in other canyons along the Front Range, debris fans are widely used in the Big Thompson area for building sites. They offer open ground with easy ac- cess, but they are vulnerable to flash flooding, and many structures above any conceivable main-stem flood were damaged or destroyed by torrents from minor side gulches. VEGETATION Vegetation was important in the Big Thompson storm and flood in two different ways: (1) as ground cover it modified the way the downpour and flooding affected the land surface, and (2) as a large component of the transported debris—trees, logs, limbs, branches, and uprooted shrubs—it modified and even intensified the damage effects. For example, open well-sodded grassy slopes withstood the impact of the storm in the heaviest downpour areas without sustaining serious erosion, but where the grass cover had been broken, as along a dirt road or trail, severe gullying was common. In pine forests, where the ground cover beneath the trees was thin, sheetflooding carried away large amounts of soil; whereas in overbank areas along the major streams, trees and shrubs slowed the current and acted as natural sediment traps. Uprooted vegeta- tion carried along by the flood rammed the sides of buildings, clogged and pushed aside culverts and bridges, and immensely complicated the tasks of salvage and cleanup operations. The storm and flood straddled the boundary of two major vegetational regions: the plains grassland region and the montane forest (Marr, 1964, p. 36—39). These regions have resulted from climatic differences that in turn are the result of varied physiography and altitude. Thus, the plains grassland coincides with the Colorado Piedmont section of the Great Plains physiographic province, and the montane forest coin- cides with the lower mountain section (the foothills by some definitions) of the Southern Rocky Mountains (Crosby, 1978). Each of the vegetational regions con- tain many different plant communities (Marr, 1964, table 1; 1967). PLAINS GRASSLAND The plains grassland east of the mountains has been greatly modified in the past hundred years or so by cultivation and grazing. Some of the area inundated by the flood was planted in cash crops, such as corn and sugar beets, but along the banks of the Big Thompson River uncultivated expanses of grassy meadow are in- terspersed with brush thickets and cottonwood groves; these areas bore the brunt of the flooding east of the mountains. Close to the mountains are isolated trees and local stands of ponderosa pine and Rocky Moun- tain juniper, some of which—on the banks of the river—were carried away by the flood. Sedimentation predominated over erosion along the flood plain, although there was considerable scour on the outside bends of some meanders. Part of the flotsam carried along and redeposited by the flood was timber uprooted by the river, but part of it was old deadfall material buoyed up and floated off by the current. MONTANE FOREST Upstream in the mountains is the montane forest, which is divisible into lower and upper zones (Marr, 1967, p. 25, 39). These zones have distinctive characteristics, but the boundary between them is in- distinct. Most of the affected area was in the lower montane forest, which normally extends up to an altitude of about 8,000 ft; ponderosa pine predominates, but this forest varies widely in character, depending on slope aspect, soil moisture, and human use. Climax grassland occupies areas of thick loamy soil in places, such as Piper Meadows. On dry rocky south-facing hillsides Rocky Mountain juniper is interspersed with scattered ponderosa pine (Marr, 1964, p. 38; 1967, p. 27—28). Forest cover is much denser on north-facing than on south-facing slopes. Vegetation is sparse under the trees and does not completely cover the ground. Such bare ground is very vulnerable to sheet erosion. Along the streamsides and moist valley bottoms are groves of narrow-leaf cottonwood, willow, and river birch; ponderosa pines dot the well-drained gravel ter- races. Many of these plants, including trees probably over 100 years old, were ripped out by the 1976 flood. Those that remained standing trapped large quantities of debris. On more northerly exposures, Douglas-fir competes favorably with the pines, extending from river level to the top of the zone in dense and nearly pure stands. The upper montane forest lies generally above 8,000 ft and, hence, includes only the higher mountain tops GEOLOGIC AND GEOMORPHIC EFFECTS, BIG THOMPSON CANYON AREA 95 in the study area. Douglas-fir predominates. This zone was little damaged by the July 31, 1976 storm. GEOMORPHIC EVIDENCE OF PAST FLOODING Geomorphic evidence supports the certainty of in- tense past flooding and, by inference, the probability of future flooding on Front Range streams. Most canyons along the Front Range, including Big Thomp- son Canyon, are physiographically similar and have had parallel geologic histories; all share the orographic controls that led to the July 31, 1976, cloudburst. Nearly all Front Range canyons in the lower montane zone (below 8,000 ft) bear evidence of intense channel scour in geologically recent time. Some of this evidence is historic and has involved the destruction of segments of roads and railroad grades. Large debris fans also are common in nearly all canyons in the lower montane zone, and they are clearly the products of tor- rential runoff; some of them have been modified by historic flash flooding. Morphologically, they are close counterparts of fans that were involved in the 1976 flooding of the Big Thompson. Big Thompson Canyon itself contains many such fans that predate the 197 6 flood. Finally, the channels of most Front Range streams contain bouldery flood gravels analogous to those transported and deposited by the Big Thompson flood of 1976. GEOLOGIC AND GEOMORPHIC EFFECTS OF THE FLOOD As flooding poured from the tributarties, the Big Thompson rose rapidly toward record discharge. The geologic and geomorphic effects of the 1976 flood are summarized in table 6. Figure 64 is a schematic representation of flood effects in various parts of the Big Thompson Canyon. The surging flood caused widespread changes in channel geometry and in dis- tribution and texture of sediment as the river eroded its banks, scoured new channels cut off meanders, re- worked its bed material, and deposited a vast amount of sediment. The river accomplished more erosion and deposition than it normally does in years, decades, or even longer periods. Many tributaries in the down- pour area scoured rapidly down to bedrock through thick surficial fills; total degradation, expressed as transported sediment, was many thousands of cubic yards. James Balog (1977) estimated that the major tributaries in the area of heavy precipitation yielded about 265,000 yd3 of sediment; this figure omits scour along the main drainages of the Big Thompson and North Fork and scour caused by sheet erosion. Very little bedrock actually was eroded, but because of the removal of surficial material by channel scour and sheet erosion on hillslopes, much bedrock was newly exposed to further weathering and erosion. BIG THOMPSON RIVER AND TRIBUTARIES FROM ESTES PARK TO DRAKE ESTES PARK AREA VALLEY SLOPES Geologic effects of the July 31, 1976 storm in the Estes Park area increased toward the east and de- creased rapidly toward the west (pl. 2A). This distri- bution reflects the strong precipitation gradient, which ranged from about 1 in. at the town of Estes Park to about 10 in. near Olympus Heights (pl. 2A). Sheet ero- sion was most intense on forested slopes on the east side of Dry Gulch and on the south flank of Mount Olympus. Nearby gravel roads and nonvegetated ar- tificial fills were also eroded by storm runoff. Sheet erosion was negligible in areas of continuous, un- disturbed grass cover, even though the grass was laid flat in many places by the surface flow. In steep grassy areas, however, the runoff locally scoured small, widely spaced depressions in the dense turf. Along some inter- mittent streams the grass was reduced to stubble by the abrasion from the sediment-laden floodwaters. Sheetwash from eroded slopes formed thin veneers of fine to coarse sand that contained scattered pebbles and cobbles (fig. 65) commonly alined downslope. These deposits accumulated chiefly at the base of grassy hillslopes and on valley bottoms and were usually less than 4 in. thick. Sand sheets as much as 1—2 ft thick were deposited locally against obstruc- tions such as fences and buildings. Cobbly to bouldery debris was deposited at the mouths of some of the small gullies that headed on steep forested slopes (fig. 66). Unimproved dirt roads, particularly those on or in- tersecting grass-covered natural drainageways with gradients of more than 10 percent, were extensively gullied. Runoff along shallow-rutted dirt roads eroded narrow steep-sided gullies as much as about 600 ft long, 7 ft deep, and 20 ft wide. Most gullies gradually deepened upslope and terminated abruptly in steep headcuts. Drainage ditches along steep roads were eroded to bedrock in many places. Mass wasting ef- fects were limited to the south flank of Mount Olym- pus, near Crocker Ranch, where two small landslides developed on slopes greater than 65 percent. 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X X X X X X X X X X X Dirt roads and . . X X NA NA X X X X NA X NA X NA NA NA Gullyin g modified slopes. Natural slopes ..... X X X X X X X X X X I: Debris Slides ...... X X X X X X X m .9 5‘ E Rockslides ........ X X X W m Landslidmg Debris flows ....... X X X Rockfalls and (or) : debris avalanches. X X X X 3% Lag deposits and (orb sheetwash ............ X X X X X X X X X X X m c E Landslide deposits ........................ X X X X Stream and (or! - X X X X X X X X X Lateral cutting Mslope zepfiilts‘ g: anma e f' s and '3, embankments. X X X X X NA NA X NA NA NA NA NA NA NA NA D $3 Channel-bank slumping ................... Channel scour ............................ X X X X X X X X X X X X X X X Boulder gravel ..... X X X X Boulder to cobble gravel. X X X X X X X X X X X Cobble to Channel fill pebble gravel. X X X X X X X X X X X X Pebble gravel ...... X X X X X X X X X X X Pebbly sand ....... X X X X X X X X X X X m G) U E Coarse sand ....... X E Medium to 3 fine sand. 1: g Debris fans at mouths of gulches ........... NA NA NA NA NA X X X X X X .E' n a 3% Debris flows at mouths of gulches .......... NA NA NA NA NA X D) '8 S. a 5 Boulder gravel ..... X '35- Boulder to E cobble gravel. X X X a Cobble to 4: X X X E Overbank sediments pebble gravel. g Pebble gravel ...... X X x Pebbly sand ....... X X X X X Coarse sand ....... X X X X ' X Medium to X fine sand. Fine sand to silt. X Silt ............... Large deposits of bouyant debris ........... X X X Average gradient (percentl .......................... 1.1 1.6 6.7 3.1 3.2 3.3 9.1 11.8 8.1 6.8 9.6 13.3 10.0 15—35 15—40 NA NA Peak discharge Iftjls) ............................... 4.330 28,200 4,460 8,700 7,210 6,910 3.540 5,500 Velocity {ft/s) ...................................... 8 22 12 26 28 21 13 19 tributaries from Olympus Dam to the confluence with the South Platte River, including average gradients and selected hydrologic data GEOLOGIC AND GEOMORPHIC EFFECTS, BIG THOMPSON CANYON AREA were minor and of very limited extent; blank spaces for hydrologic data indicate that data was not collected] 99 North Fork Big Thompson River and tributaries from above Glen Haven to confluence with the Big Thompson River North Fork to canyon mouth Big Thompson River from confluence with the Big Thom son River from can- yon mout to confluence with {mile 10.0 to mile 0.0) (mile 44.9 to mile 37.0) the South Platte River (mile 37.0 to mile 0.0! North Fork Big Thompson River Major tributaries Minor tributaries 5; EA: J: In 3 o -S E E"; I .— at 3.3 a — G) E f: = 5 '8 2 1: E Unnamed CD or or "‘ E a E I- > 'U 9 3 fig :33 E3 e5 5 9-: <5 .5 :2 8 D 5 a: Mile Mile Mile Mile Mile Mile Mile Mile Mile Mile Mile Mile Mile Mile Mile Mile Mile Mile Mile Mile Mile Mile Mile 10.0- 8.34 6.7- 5.0- 3.0— 8 5_ 8 4 7 3_ 6 5_ 5 6— 10.0- 6.5- 1.5- 44.9- 44.1- 41.3— 41.0— 40.4— 38.9- 37.0— 34.3— 25.0- 15.7— 8.3 6.7 5.0 3.0 0.0 ' ' ' ' 6.5 1.5 0.0 44.1 41.3 41.0 40.4 38.9 37.0 34.3 25.0 15.7 0.0 X X X X NA X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X NA NA NA NA NA X X X X X X NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA X NA NA NA NA NA NA NA NA NA NA X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X 3.7 2.6 1.9 .5 1.8 3.8 g; 12.1 8.2 14301 15-35 10—40 NA 1.4 2.7 2.5 2.5 0.8 2.3 1.3 0.4 0.2 0.2 2.810 1,990 2,470 890 8,710 1.300 2320 9.670 2.060 30.100 31.200 Mile ()9 8 12 9 12 29 11 16 7 12 26 100 FLOOD, JULY 31—AUGUST 1. 1976, BIG THOMPSON RIVER, COLORADO FIGURE 65.—Helicopter view showing effects of sheetflooding along foot of Mount Olympus at Crocker Ranch. Thin layers of sandy sheet- wash (light tones). Bouldery gravel (darker tones) at mouth of gully in upper left of photograph. This area received about 10 in. of rain. DRY GULCH Dry Gulch, which had a peak discharge during the storm of 4,460 ft3/s and a velocity of 12 ft/s (Part A, table 4), was slightly affected by minor channel scour, lateral cutting, channel aggradation, and overbank deposition. Downcutting appears to have been most in- tense just upstream from Eagle Rock, where the average gradient of the gulch increases from 2.4 to 5.0 percent. Downstream from this point, sandbars ac- cumulated at the insides of bends, and the floor of the gulch was partly filled with coarse pebbly sand. Lateral cutting along the outsides of bends removed a small amount of bank material and trimmed the toe of Olympus Dam (fig. 67). Culverts and roadfill in the gulch were washed away. At one point, the stream abandoned a short section of manmade channel and reoccupied its original course. At the confluence with the Big Thompson River, sediment from Dry Gulch built a large alluvial fan that partly filled the stilling basin at the base of Olympus Dam. Between the dam and the head of the canyon, floodwaters from Dry Gulch deposited point bars at the insides of bends and a small amount of sandy overbank alluvium on low ter- races. BIG THOMPSON CANYON, OLYMPUS DAM TO DRAKE SHEET EROSION AND GULLYING The Big Thompson Canyon and its tributary gulches above mile 50 received in excess of 10 in. in rainfall during the storm (pl. 2A). On steep forested slopes, in- tense runoff and saturation of the surficial mantle caused widespread erosion. Sheet erosion locally removed the thin layer of organic litter and as much as a foot of pebbly sand from the underlying colluvium, leaving thin lag GEOLOGIC AND GEOMORPHIC EFFECTS, BIG THOMPSON CANYON AREA 101 deposits of pebble- to boulder-size rock fragments on eroded slopes. Slopes with subtle surface irregularities and a sparse understory of grass were the most suscep- tible to sheet erosion. Erosion was much reduced by large tree roots, which helped stabilize the substrate and trapped pebbly sheetwash on their upslope sides. Sheetwash deposits also accumulated on the upslope sides of large boulders. Logs and other obstructions together with accumulating sediment deflected the surface flow and concentrated its erosive power in unobstructed areas. Small alluvial fans and thin alluvial aprons formed at the bases of eroded slopes (fig. 65). Surficial deposits along minor drainageways were gullied to as deep as 5 ft. Channelized runoff commonly scoured the surficial mantle down to bedrock and car- ried away material as large as 16 in. in intermediate diameter. Natural drainageways containing unpaved roads were the most extensively gullied. LANDSLIDING Rapid mass movements, including debris slides, rockslides, debris flows, and rockfalls, were activated between miles 56.2 and 47.7 on northeast- to northwest- to southwest-facing slopes where the in- clination was from about 60 to 85 percent. Debris slides and valley-side debris flows generally developed in thin colluvial mantles, less than about 6 ft thick, that are composed of angular rock fragments in a matrix of slightly silty sand (fig. 68). These colluvial mantles overlie bedrock slopes that tend to be parallel to the present topography. Slope failures developed at or near the colluvium-bedrock contact, and most of them were probably caused by rapid infiltration of pre- cipitation, which increased the weight and decreased the shear strength of the surficial material by in- creasing the pore-water pressure within the mass. Some debris sliding, however, was caused partly by lateral stream cutting at the bases of steep slopes FIGURE 66.—Tongue of bouldery debris deposited by sheetflooding along the base of Mount Olympus at Crocker Ranch. This is a ground view of the area shown in the upper left part of figure 65. The two large boulders on the right in the middle distance were not moved. 102 FLOOD, JULY 31—AUGUST 1. 1976, BIG THOMPSON RIVER, COLORADO FIGURE 67.—Toe of Olympus Dam, scoured by floodwaters of Dry Gulch, viewed toward the south. along the outsides of bends (fig. 108). Most debris slides ranged in volume from a few tens to several hun- dreds of cubic yards. At several localities between miles 56.2 and 47.7, small boulders to blocks of rock as large as about 30 ft across were set into motion down natural and modified slopes. Most rockslides occurred either at building sites where colluvium had been excavated to provide additional building space (fig. 69) or at steep highway cuts. Colluvium near mile 56.1 and surficial deposits in some of the small tributary gulches between miles 52.2 and 49.9 were mobilized and redeposited during the storm as debris flows. Debris flows that developed on colluvial slopes moved down minor drainageways. These flows were long and narrow, and they were flanked by prominent levees 1—3 ft high that formed when the flanks of the flows stabilized while the cen- tral parts continued to move. The levees consisted of angular clasts as large as 1.8X5.1 X 5.2 ft in a matrix of very slightly silty sand. The channels between the levees contained thin deposits of pebbly to bouldery material. Trees in the paths of debris flows were scar- red by impacts on their upslope slides to a height of about 5 ft. Debris flows deposited at the mouths of small tributary gulches were lobate, steep sided, and without distinguishable levees. The flanks and centers of these flows appear to have come to rest at the same time, so levees did not form. The deposits ranged in thickness from about 3 to 9 ft and consisted of pebbly to bouldery sand. The sides and toes of some debris flows were strewn with broken tree trunks, as much as 16 in. in diameter that were stripped of limbs and bark, along with boulders as large as 2.1X3.2X3.6 ft. Most debris flows were partly covered by a thin layer of coarse- grained water-laid sediment. Bouldery debris thought to have been deposited by highly sediment charged GEOLOGIC AND GEOMORPHIC EFFECTS, BIG THOMPSON CANYON AREA 103 FIGURE 68.—Small debris slide in stony colluvium on 67-percent slope near mile 53.2 This area received about 11.5 in. of rain. runoff accumulated at the mouths of some of the small tributary gulches (fig. 103). These deposits lack the characteristic features of debris flows, although they appear to be of somewhat similar origin. Rock fragments dislodged from numerous cliff faces fell and bounded down steep slopes during the storm. The largest rockfall was near the west end of Glen Comfort, where falling granite slabs measuring as large as 2.3X6.0><6.4 ft splintered mature ponderosa pines and Douglas-firs growing at the base of the slope. At other localities, airborne rock fragments as large as 1.1 X 2.3 X 3.5 ft broke off trees as much as 9 in. in diameter and produced impact scars as high as 5 ft on the uphill slides of larger trees. MAJOR TRIBUTARY GULCHES Major tributary gulches between Loveland Heights and mile 49.6 (pl. 2A; table 6) were extensively scoured during the storm. Near the center of maximum precipitation, two gulches with average gradients of 9—12 percent—the unnamed gulch at the east end of Loveland Heights (mile 54.4) and Dark Gulch—were stripped of surficial material for distances of as much as a mile. Side gulches along these major tributaries were also deeply eroded. Granite bedrock along the main gulches was exposed at depths of about 2—8 ft below the level of the preflood channel. A few bouldery deposits, however, were laid down at the insides of sharp bends; boulders larger than 5 ft in diameter were not moved. The amount of lateral scour generally in- creased downstream. Postflood channel width ranged from about 15 to 35 ft along Dark Gulch and was as much as about 90 ft near the mouth of the unnamed gulch. Both gulches had peak discharges in excess of 7,000 ft3/s and velocities of 26—28 ft/s (Part A, table 4). Surficial deposits exposed by scour in streamcuts in the tributary gulches consist 'mostly of sandy slope wash over pebbly to bouldery flood gravels (fig. 104 FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO FIGURE 69.——Motel unit damaged by a rockslide on an oversteepened bedrock slope near mile 56.0. Movement was along a sloping joint plane in granite. This area received about 10 in. of rain. FIGURE 70,—Unnamed gulch, viewed about 100 yd upstream from its confluence with the Big Thompson River, at mile 54.4. Floodwaters with a calculated velocity of 26 ft/s and a peak discharge of 8,700 ft3/s (Part A. table 4) scoured the gulch to bedrock, exposing sandy slope wash and an older bouldery flood deposit. Before the 1976 flood, this part of the gulch was a flat-bottomed grassy meadow without perennial surface drainage. GEOLOGIC AND GEOMORPHIC EFFECTS, BIG THOMPSON CANYON AREA 70). In most exposures, the slope wash is humic throughout and appears to be a cumulative A horizon that is at least a few hundred years old. The underlying alluvium may date from the last major flood. the preservation of the buried alluvium suggests that it was deposited by a flood of lesser magnitude than the 1976 flood. Unimproved dirt roads on grassy slopes of about 7 percent along the lower reach of the unnamed gulch at Loveland Heights and in the large meadow in its head- waters were gullied to depths of as much as 5—10 ft (fig. 71). Roads parallel or oblique to the contour of the ground were less deeply gullied than those perpen- dicular to the contour. Flood debris along the unnamed gulch and Dark Gulch was deposited at heights of about 7—9 ft above the present streambed. Major tributary gulches with average gradients of less than 10 percent, peak discharges of less than 7,000 ft3/s, and velocities of less than 25 ft/s (Part A, table 4)—including Noels Draw, Rabbit Gulch, and Long Gulch—were less severely eroded than gulches of com- parable size with similar or steeper gradients, higher peak discharges, and higher velocities. The lower half a 105 mile of Noels Draw (fig. 1013) and its side gulches slightly farther upstream were scoured to bedrock. The depth of scour was about 1—7 ft on the side gulches and about 5—6 ft on the main gulch. Upstream from this segment of N oels Draw, the gulch was eroded and part- ly backfilled with pebbly to bouldery gravel in which the floodwaters later cut braided channels. Transported boulders were as large as 3.0X3.5X4.5 ft. The height of flood debris above the present floor of Noels Draw ranged from about 8.5 ft in the lower 500 ft to about 6 ft above the confluence with Solitude Creek. Cobbly tunnel tailings in N oels Draw, about 0.9 mi above the mouth, were gullied to depths of about 6—15 ft by the discharge from a small tributary gulch. The sparse vegetation on the tailings provided little protec- tion against erosion, and coarse rock debris from the tailings pile accumulated in alluvial fans at the mouths of the eroded gullies. Debris slides were set off along the flanks of the tailings pile by lateral stream cutting. Elsewhere in N oels Draw and its side gulches, debris slides developed in thin layers of saturated colluvium resting on steep north- to west-facing bedrock slopes. FIGURE 71.—Gtu cut by intense runoff channeled along the shallow ruts of a dirt road in a grassy area, upper part of unnamed gulch at mile 54.4 Rainfall here was about 10 in. 106 The largest slides were along the upper part of Solitude Creek. Long Gulch was scoured to depths greater than 6 ft and was partly backfilled with coarse gravel more than 3 ft thick. The lower ends of most side gulches were cut to bedrock. Old debris fans at the mouths of small side gulches were truncated by the floodwaters of the main gulch, exposing boulders as large as a few yards across. Bouldery gravel bars were deposited along in- sides of bends. Stranded driftwood lay 8 ft above pre- sent drainage a mile upstream from the mouth of the gulch and 11 ft above drainage at the mouth. MINOR TRIBUTARY GULCHES Minor tributary gulches between miles 55.6 and 50.9 were severely scoured; many of them to bedrock, through 2—3 ft and locally as much as 6 ft of surficial material. The postflood width of channel scour ranged from about 10 to 30 ft and generally increased with the size of the gulch. Boulders as large as 1.7X3.0><4.5 ft as well as finer grained sediment were removed by the floodwaters. Some gulches were stripped to bedrock for distances of as much as a quarter of a mile, although most contained discontinuous flood deposits. Coarse gravel deposits with boulders as much as 3 ft across accumulated to a thickness of as much as 13 ft against massive log jams and to a thickness of about 3—5 ft behind large boulders, mature standing pine trees, and fallen trees. Deposits with elongate rock fragments commonly displayed imbricate structure. Small sandbars were deposited at the insides of bends and in other protected areas. In narrow bouldery gulches, smaller boulders jammed behind larger ones created shallow basins filled with pebbly to bouldery sand. Undercutting and sliding of slope material con- tributed considerable sediment to the flood-swollen gulches. Some boulders as large as 8 ft in intermediate diameter were undercut and transported a few yards downstream. In the steeper reaches of some gulches, sand- to small boulder-size material was washed away, leaving the floors of the gulches mantled with chaotic lag deposits of massive boulders. The height of flood debris above the present floors of many minor gulches varied from about 5 to 8 ft and increased downstream. BIG THOMPSON RIVER OLYMPUS DAM TO LOVELAND HEIGHTS In the upper 2.2 mi of the canyon, the Big Thompson River lacked the volume of water and competence to cause extensive erosion and damage. Along this reach lateral cutting was restricted to outsides of channel bends where small volumes of roadfill were removed and a house founded on artificial fill was severely FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO undercut. Near the head of the canyon, the Big Thomp- son River moved small boulders as large as 1.0X1.2><1.3 ft (table 7). Farther downstream, small sandbars were deposited at insides of bends. Below mile 55.7, the decks of most private bridges were washed away, and their concrete center piers were rotated by local scour that cut to depths of at least 1—2 ft. Low stream terraces in the vicinity of Loveland Heights were overtopped by the floodwaters and were covered by a thin layer of sand. TABLE 7 .—Size and lithology of largest boulders transported by the 1976 flood in the Big Thompson Canyon, mile 56.4 to mile 54.7 [River mile locations are shown on plate 1 ] River mile Location Size (feet) Lithology 56.4 Near head 1.0x 1.2x 1.3 Granite. ofcanyon. 0.5x 1.1x 1.8 Do. 0.5x 0.6x 0.9 Do. 54.0 0.7 mile 1.5X 2.3x 2.8 Granite. upstream 1.2x 1.9x 2.5 Do. from Glen 1.2x 1.7X 3.0 Do. Comfort. 1.1X 1.7x 3.3 Do. 1.1X 1.3x 3.0 Do. 53.3 At upstream 2.8x 3.0x 10.7 Granite. end ofGlen 2.6x 3.2x 6.0 Do. Comfort. 2.5x 3.8x 6.0 Do. 2.3x 3.0x 7.1 Do. 2.0x 4.8x 5.1 Do. 52.3 Near downstream 1.6x 2.3x 4.3 Granite. end ofGlen 1.3x 2.3X 3.0 Do. Comfort. 1.3x 1.9x 2.8 Do. 48.1 1.1 mile upstream 6.0x 6.5X10.0 Pegmatite. from 6.0x 6.0>< 9.0 Do. Waltonia. 4.4x 5.7><12.0 Granite. 3.5x 8.6X12.3 Do. 3.0x 9.0X19.5 Pegmatite. 46.4 1.0 mile 1.8x 2.0X 2.2 Pegmatite. downstream 1.4x 1.5x 2.6 Do. from 1.3x 1.9x 2.1 Do. Waltonia. 1.2x 1.7>< 2.0 Do. 1.0x 1.6x 2.2 Do. 45.7 0.5 mile 7.3X 8.0x 9.0 Granite. upstream 4.8X 5.3x 9.7 Do. from Drake. 4.0x 8.3x 9.7 Do. 3.5x 6.1x 8.5 Do. 2.6X 8.0x 9.0 Do. Upstream from Loveland Heights, the Big Thomp- son River received a small amount of sediment from the valley sides and from tributary gulches. A major source of sediment was a debris slide at mile 56.1, which released a few hundred cubic yards of colluvium into the main stream during the storm. Alluvium was GEOLOGIC AND GEOMORPHIC EFFECTS, BIG THOMPSON CANYON AREA deposited mostly above the mouths of most gulches; only minor amounts entered the Big Thompson River. The gulch at mile 55.6, however, contributed a signifi- cant amount of sediment to the mainstream. A bouldery fan accumulated at the mouth of the gulch, and gravel bars extended down the Big Thompson River for a distance of about 300 ft. LOVELAND HEIGHTS TO DRAKE Debris-charged floodwaters from major tributary gulches between miles 54.4 and 49.6 transformed the Big Thompson River into a raging torrent that caused extensive channel modification and widespread destruction all the way from Loveland Heights to the mouth of the canyon. The Loveland Heights area was near the center of the downpour (p1. 2A), and much of the storm runoff entered the mainstream along this reach. Between the east end of Loveland Heights and Noels Draw, relatively minor lateral cutting washed away a small amount of roadfill. Downstream from 107 Noels Draw, however, lateral scour along outsides of bends completely removed short segments of the highway and, at mile 52.4, destroyed a house and re- activated a large debris slide (fig. 108). Near Glen Com- fort, stream-polished boulders as large as 2.8X3.0>< 10.7 ft (table 7) were undercut and carried downstream. Boulders transported during the 1976 flood were identified on the basis of one or both of the following criteria: (1) fresh impact scars on the downstream side (fig. 72), and (2) deposition on or against flood debris or surfaces previously free of sedi- ment. Channel scour to a depth of at least several feet caused minor settlement of the concrete center pier of the highway bridge at mile 52.2. Major tributary gulches funneled large amounts of coarse sediment into the mainstream. Coarse debris deposited at the mouths of the unnamed gulch at mile 54.4 and Dark Gulch created bouldery alluvial fans that partly constricted the main channel. Much of the pebbly to cobbly sediment from these gulches, FIGURE 72.—Granite boulder near mile 42.7, showing fresh impact scars. Pick is 17 in. long. Such scars on the downstream sides of boulders indicate movement during the flood. 108 FLOOD, JULY 31—AUGUST 1, 1976. BIG THOMPSON RIVER, COLORADO FIGURE 73,—Overbank sand and buoyant debris on the south side of the Big Thompson River at mile 54.4. Frame house in center of photograph was lifted off its foundation and pushed against trees. Much of the damage here was caused by a surge of water from the tributary on the opposite (north) side of the river. FIGURE 74.—Debris pile on the upstream side of highway bridge at mile 52.2. GEOLOGIC AND GEOMORPHIC EFFECTS, BIG THOMPSON CANYON AREA however, accumulated a short distance downstream in gravel bars that were deposited on low islands and at inner bends. The tops of most gravel bars were washed free of sand and were cut by shallow cross channels. Coarse pebbly sand partly filled the main channel as far downstream as half a mile below the major gulches. Thin layers of sand to silty sand were deposited on low forested terraces along inner bends where the vegeta- tion impeded the overbank flow. Floodwaters from the unnamed gulch deflected the mainstream flow against the opposite bank where a house was floated off its foundation and deposited a short distance downstream (fig. 73). Piles of floating debris commonly ac- cumulated in wooded areas along the river and on the upstream sides of highway bridges (fig. 74). From the mouth of Long Gulch to mile 48.3 bouldery gravel bars were deposited on the insides of bends; small amounts of roadfill and slope material were washed away, primarily on the outsides of bends. At mile 48.3, the stream gradient increases from an average of 1.3 percent to 6.7 percent. This section of the canyon, about 3,700 ft long, was deeply scoured along its entire length. Boulders as much as 9 ft in in- termediate diameter were transported downstream (table 7), locally exposing the underlying bedrock and leaving the channel lined with massive blocks of granite and pegmatite together with a minor amount of bouldery gravel. Lateral erosion removed short segments of highway; undercut colluvial slopes col- lapsed into many small debris slides. Between mile 47.7 and the confluence with the North Fork at mile 44.9 the stream gradient is relatively uniform and averages 3.1 percent. Along this reach lateral erosion removed about 1 mi of highway. The longest segments of intact road were preserved at the insides of bends. Three old debris fans that extended an estimated 40—50 ft into the canyon, between miles 45.4 and 46.3, were washed away by the flood (fig. 75), as was a riverside motel complex at Waltonia (mile 46.9; fig. 76). Considerable channel scour took place along a 1,000-ft segment of the Big Thompson near mile 46.3, where the gradient abruptly increases from about 2.9 to 6.7 percent. Widespread coarse flood gravel was deposited in the main channel and on low terraces below mile 47.6. Large gravel bars accumulated along the insides ,of bends and along relatively straight and slightly ex- panded reaches. The surfaces of most of these gravel bars were mantled with boulders as large as 7.3X8.0><9.0 ft (table 7). Shallow channels cut diagonally across the bars. These bars were as much as 9 ft thick, and they displayed crude horizontal stratification. Sandbars accumulated outside the main 109 channel in overbank areas of lower flow velocity, such as the downstream sides of closely spaced houses. These bars were as much as 3 ft thick and consisted of micaceous, thinly bedded, horizontally stratified very fine to very coarse sand. The flood caused widespread property damage at Drake (fig. 77). At the south end of town the river abandoned its preflood course and cut a new channel along a gravel road on the east side of the canyon. Then a bouldery gravel bar was deposited at the upper end of the new channel, partly filling it (fig. 78). This obstruction deflected the floodwaters against the west wall of the canyon where a second channel was out along U.S. Highway 34. Coarse flood alluvium deposited by a series of shallow distributary channels along the eastern margin of the second channel com- pletely filled the preflood channel. BIG THOMPSON RIVER FROM DRAKE TO THE CANYON MOUTH Many canyon occupants were caught by surprise on the night of July 31, 1976, because little or no rain had fallen within this stretch of the canyon. Many buildings, mobile homes, campers, and cars were destroyed—some with their occupants inside. At Drake the electric power was lost and heavy rain was known to be falling upstream, but many residents still were unaware of the impending danger. The flooding at Drake, first from the Big Thompson River at about 2100 MDT, then about half an hour later from the North Fork, took at least 13 lives and destroyed many buildings and other properties. Some buildings were merely shifted off their foundations, but others were swept away (fig. 79). Buildings that still stood were partly filled with sediment, and most of them were damaged to the extent that they had to be demolished. Extensive structural damage was caused by impact from floating and saltating debris and by hydraulic pressure. Most structural failures were near points of weakness, such as windows and doors (fig. 80). Scour was severe along most of the river channel and along many parts of U.S. Highway 34. Boulders, one as large as 3,200 ft3, were moved by the current. The concrete dam below Midway was destroyed. Sediment deposited in the flood plain contained all sizes of material from sand to bouldery gravel. The tributaries along this stretch of the canyon were outside the area of heavy rainfall and added nothing to the flood. As the flood moved down the canyon, erosion and deposition were influenced by the gradient of the river, the sinuosity of the canyon, and the width of the flood plain. Reaches steeper than 2 percent were scoured, especially on the outsides of bends and where the chan- nel was constricted. Deposition took place where the 110 FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO FIGURE 75.—House undercut by Big Thompson River near mile 45.4. House was located on a debris fan at the mouth of the small gulch in background. Rainfall here was relatively light, and the gulch carried no water of any consequence. gradient decreased to less than 2 percent, at wide places on the flood plain, and at the insides of bends. Deposition of large boulder bars diverted the river from its preflood channel. Deposition was intensified where the flow was impeded by bridges, buildings, and dense vegetation. DRAKE TO MIDWAY From Drake (mile 44.9) to Midway (mile 44.0), the canyon coincides with the Thompson Canyon fault zone; and the course of the river, therefore, is relatively straight, the valley is broad, and the gradient is only 1.6 percent. Channel modifications by the flood were moderate along this reach, except that the highway was destroyed along the outside of the bend just upstream from Midway. Boulders deposited in this reach of the main channel were as long as 3.9 ft (table 8, mile 44.3). Pebbly sand to cobble gravel was deposited in overbank areas. At Midway, the flood reportedly rose from normal flow to flood stage in less than 5 min. Here, the gra- dient steepens from 1.6 percent to locally more than 3 percent, and scour intensified along the winding chan- nel. At the outsides of bends, lateral scour triggered some small landslides and destroyed short segments of the highway. Inside of bends were aggraded with bouldery alluvium. A bouldery gravel bar about 9 ft thick was deposited on a low terrace on the inside of the bend just west of Midway (mile 44.1). Boulders on this bar were as much as 6.4 ft in longest dimension; some of the larger boulders probably were scoured from a road fill about 450 ft upstream. MIDWAY TO CEDAR COVE In the 4 mi between Midway (mile 44.0) and Cedar Cove (mile 40.0), scour removed about 40 percent of the highway and destroyed many manmade structures, in- cluding a diversion dam, a restaurant and motel, and a GEOLOGIC AND GEOMORPHIC EFFECTS, BIG THOMPSON CANYON AREA 1 1 1 FIGURE 76.—The riverside community of Waltonia, mile 47.0, before and after the 1976 flood. Approximate scale: 1 in.=200 ft. A, Preflood view. Most of Waltonia is built on a large debris fan. Prior to 1976 flood, US. Highway 34, two large motels, and several other buildings were located here in Big Thompson flood plain. B, Postflood View, August 3, 1976. Virtually all of Waltonia on flood plain was obliterated. powerplant. Along most of this stretch the river than in stretches just upstream and downstream. swings away from the Thompson Canyon fault zone, Locally, the gradient is as steep as 4 percent. and the canyon floor is generally narrower and steeper The dam below Midway at mile 43.25, was about 25 1 12 FLOOD, JULY 31—AUGUST 1, 1976. BIG THOMPSON RIVER, COLORADO GEOLOGIC AND GEOMORPHIC EFFECTS, BIG THOMPSON CANYON AREA { FIGURE 77.—Helicopter view showing flood damage in part of Drake. Downstream (north) is at top of picture. Before the flood the chan- nel of the Big Thompson River was just to the left of the houses, but it shifted widely during the flood and was far to the left out- side the picture at the time the photograph was taken. Rainfall here was about 4 in. ' TABLE 8.—Size and lithology of largest boulders transported by the 1976‘ flood in the Big Thompson Canyon, mile 44.3 to mile 39.5 [River mile locations shown on plate 1] River mile Location Size (feet) Lithology 44.3 0.3 mile 1.9x 2.2x 3.9 Granite. downstream 1.8X 2.0x 3.9 Do. from Drake. 1.4x 2.3x 3.5 Do. 1.3x 1.3x 3.9 Do. 44.1 Upstream end 4.7x 5.4x 5.8 Pegmatite. ofMidway. 4.1X 5.2x 5.9 Do. 4.0x 4.2x 4.7 Granite. 3.5x 5.5x 6.4 Pegmatite. 2.7x 3.2x 6.2 Do. 43.3 Upstream side 5.0>< 5.3x 8.3 Granite. of hydroelectric 4.9x 5.0>< 7.3 Do. diversion 3.9x 4.9X 6.0 Do. dam. 2.9x 3.9x 6.0 Do. 2.9x 3.2x 8.2 Do. 2.8x 4.7x 6.0 Do. 43.2 Downstream 11.8X 12.0 X 22.9 Granite. side of 6.5x 7.0x 9.5 Do. hydroelectric 6.0x 7.2X12.5 Do. diversion 5.0>< 5.3x 8.0 Do. dam. 4.5x 5.5x 7.0 Do. 4.0x 5.0x 7.0 Do. 42.6 Covered Wagon 5.4x 7.3X10.5 Granite. Restaurant 4.5x 4.9x 7.2 Do. area. 4.3x 5.5x 8.3 Do. 4.0x 4.0x 7.0 Do. 3.9x 6.7X10.0 Do. 41.2 Loveland 3.2x 3.3x 4.9 Granite. power 2.3x 2.7x 4.4 Gneiss. plant. 2.1x 2.8x 3.2 Granite. 1.8x 2.3x 3.1 Do. 0.9x 1.7>< 3.6 Gneiss. 39.5 Cedar Cove 2.2x 2.6x 3.7 Granite. area. 1.7>< 2.8>< 3.8 Do. 1.6x 2.3x 3.3 Pegmatite. 1.1x 2.2x 3.2 Gneiss. 0.7X 2.2x 2.8 Do. TABLE 9.—Concrete blocks carried downstream from destroyed diversion dam at river mile 43.25 Block size Approximate weight Distance from dam (in ft“) (tons) (ft) 720 54 0 600 45 65 1,650 124 90 270 20 l 1 5 280 20 160 30 2 325 4 .3 2,400 113 ft high and 100 ft wide. It was built in the late 1920’s to divert water to the hydroelectric powerplant about 2 mi farther downstream. The flood first overtopped then rapidly breached the dam, carrying large sections of concrete and boulders downstream in a wave of water at least 23 ft above present stream level (fig. 81). Just upstream from the dam, as much as 10.0 ft of main stream gravel and at least 8.5 ft of overlying slack-water sediments were scoured out. Boulders deposited upstream from the dam were as large as 8.3 ft in longest dimension (table 8, mile 43.3). The south abutment of the dam remained in place, even though about 5.5 ft of alluvium was removed from its base. Below the dam a large newly deposited bouldery gravel bar extended about 650 ft downstream. The bar stood about 11 ft above present stream level and contained blocks of concrete as large as 1,650 ft3 (table 9) and many large boulders. The largest boulder apparently moved by the flood was 11.8X12.0X22.9 ft (table 8, mile 43.2) and weighed an estimated 275 tons. Within the 0.6-mi reach between the breached dam and the site of the Covered Wagon Restaurant, the floodwaters deeply scoured the 100—200-ft-wide flood plain. Here, the gradient was about 4 percent. Scour removed most of the highway on the north side of the canyon and parts of the aqueduct on the south side. THE COVERED WAGON RESTAURANT AREA The Covered Wagon Restaurant and parts of an ad- joining motel had stood on a low terrace on the north side of the river (mile 42.6). One canyon resident who watched the flood from high ground reported that ris- ing water first simply surrounded the buildings and trapped the occupants inside. Moments later, a surge of water carried off the restaurant and nearby buildings. When the flood subsided, a large bouldery gravel bar occupied the site. This stretch of canyon (mile 42.8 to mile 42.3) is about 200 ft wide and is relatively straight, but a winding channel was cut by the flood as the current was deflected from side to side, and bouldery gravel was deposited along the insides of bends. A gravel bar about 5 ft thick formed at the site of the restaurant, and its surface was strewn with boulders as much as 10.5 ft long (table 8, mile 42.6). Opposite the bar on the south bank, lateral scour on the outside of the bend ex- posed bouldery alluvium deposited by a previous flood. From the Covered Wagon Restaurant area downstream to the Loveland powerplant, a distance of 0.9 mi, about 65 percent of the highway was destroyed. Along this stretch the canyon is walled by bedrock, ex- cept for the scoured-out highway embankment, and deposition was limited to insides of bends and places where the current was slowed or impeded. A bouldery gravel bar was deposited just downstream from a highway bridge that remained standing. The bridge (mile 41.8), although overtopped was spared serious 114 FLOOD, JULY 31-AUGUST 1, 1976, BIG THOMPSON RIVER. COLORADO FIGURE 78 (above and facing page).—Preflood (A) and postflood (B) view taken near south end of Drake showing part of a huge boulder deposit that caused severe damage to part of the community. Houses in background were outside its path and were little damaged. Peak discharge here was about 28,000 ftS/s (Part A, table 3). damage because the flood scoured out the east abut- ment and formed a new channel. Another bar was formed at the inside of the bend just upstream from a 25-ft-wide bedrock constriction in the canyon wall at mile 41.4. Below the constriction, the canyon widens as it again coincides with the Thompson Canyon fault zone. Here, the flood spread out in the wide valley bot- tom where the Loveland powerplant had stood. LOVELAND POWERPLANT AND VICINITY The brick hydroelectric plant (mile 41.3) and a grassy picnic area shaded by large trees (fig. 82) had stood about 600 ft downstream from the bedrock constric- tion mentioned in the previous paragraph. As the flood poured out of the constriction, it spread across the valley bottom to depths greater than 10 ft above pre- sent stream level. Here, the valley is as Wide as 400 ft and has a gradient of about 2 percent. Only the founda- tion and generators of the powerplant remained after the flood, and almost all the trees were carried away as the flood shifted from its existing channel, reoccupied an old channel at the southern margin of the flood plain, and deposited a debris bar 800 ft long and 200 ft wide between the two channels. Deposition within this bar varied widely. Along the upstream 450 ft, near the site of the powerplant, about 20 in. of pebbly sand to bouldery gravel was deposited. It, in turn, was blanketed by flood debris, scattered boulders, and a thin layer of micaceous sand. On the re- maining 350 ft of the bar, large standing trees created a log jam 9 ft high that trapped coarse alluvium as much as 2 ft thick containing boulders as large as 4.9 ft GEOLOGIC AND GEOMORPHIC EFFECTS, BIG THOMPSON CANYON AREA 115 in maximum dimension (table 8, mile 41.2). Many logs were more than 2 ft in diameter. Downstream from the bar to mile 41.0, a distance of about 1,200 ft, channel scour reworked the riverbed and removed most of the finer materials and resulted in a bouldery streamway. A bouldery gravel bar was deposited above a constriction of the canyon at mile 41.0. Between mile 41.0 and Cedar Cove at mile 40.4, the river again swings away from the Thompson Canyon fault zone, the valley-bottom is narrow and sinuous, and the river is confined by a rock-walled canyon 50—150 ft wide. As the floodwater entered this dogleg part of the canyon, it rose to as much as 22 ft above present stream level. Where the canyon again flared out, diminished velocity reduced the tractive force of the flow, and a bouldery gravel bar was deposited. Erosion triggered a small rockfall and scoured the canyon-bottom alluvium. Boulders deposited at the sharp bend at mile 40.5 were as large as 5.8 ft in longest dimension. CEDAR COVE AREA Cedar Cove (miles 38.9—40.4) was one of the most densely developed areas in Big Thompson Canyon. Most of the homes here were built on low terraces along the river on a wide stretch that coincided with a part of the Thompson Canyon fault zone. Down through time, erosion along the fault zone and alluvia- tion by the Big Thompson River and Cedar Creek had built a flood plain about 500 ft wide and more than a mile long, with a gradient of only 1 percent. As the flood spread over the valley floor its velocity was greatly reduced. Much of its load of cobbly to bouldery material was deposited as a large braided gravel bar at the upstream end of Cedar Cove. Slightly farther downstream, a large quantity of pebbly sand was deposited as the flood, deeper than 10 ft above present stream level, overtopped the low banks and spread throughout the community. The mainstream channel and the outsides of bends were heavily scoured, damaging or destroying stream- 1 16 FLOOD, JULY 31—AUGUST 1, 1976. BIG THOMPSON RIVER, COLORADO FIGURE 79.—Wrecked house and other debris, including large mobile home, lodged on a damaged private bridge at mile 44.7, 0.5 mi down- stream from Drake, looking upstream. FIGURE 80.——Flood damage along the Big Thompson River near center of Drake, looking upstream. Frame house in foreground was washed off its foundation and damaged beyond repair. Exterior wall on the north side of this house was pushed out- ward around the window. Side of log cabin in background also was offset in a downstream direction. GEOLOGIC AND GEOMORPHIC EFFECTS, BIG THOMPSON CANYON AREA 117 FIGURE 81.—Remains of diversion dam below Midway at mile 43.25. Before breaching the dam, the water rose at least 5 ft above the crest and left a log wedged in the window of the shack on the right abutment. front buildings. Buildings on the flood plain were inun- dated with sediment, and some were carried away. Buildings, fences, debris piles, and trees on the flood plain all retarded the current and intensified deposi- tion (fig. 83). Especially on insides of bends, as near mile 39.5, sand accumulated to depths of more than 3 ft. This sand was horizontally bedded, micaceous, and fine to medium grained. It graded into gravelly sand toward the mainstream channel. Flood alluvium along the main channel was mostly sandy to cobbly gravel, but locally derived boulders were as long as 3.8 ft (table 8, mile 39.5). Below Cedar Cove, the floodwaters entered The Narrows. THE NARROWS At The Narrows, mile 38.9 to mile 37, the canyon is only 80—100 ft wide at river level and is walled by steep cliffs of metamorphic rock as much as 1,000 ft high. The mean gradient of the river along this reach is 2.4 percent. In The Narrows, US. Highway 34, an irriga- tion diversion system, and an overhead siphon were all destroyed by the flood. Along this stretch, the effects of flooding were distinctly different from those at Cedar Cove. Flood- water leaving the Cedar Cove area was loaded with silt, sand, and pebbly gravel, but within The Narrows it quickly picked up coarser debris. Rising 14 ft above present stream level, the flood damaged the concrete dam and diversion system at mile 38.65. It ripped out a 110-ton siphon at the canyon mouth when a floating building knocked out one of the supports (fig. 84). The road embankment, which had occupied about one-half of the canyon floor, was completely removed except for a few tens of feet along insides of bends. Deposition in The Narrows was confined to gravel bars at the insides of some bends and to small patches of gravelly sand on bedrock walls along the outsides of a few bends. Boulders on the gravel bars were derived chiefly from the roadfill and were as large as 5 ft in maximum dimension. In protected slackwater along the canyon wall, finely laminated and horizontally bed- ded fine- to medium-grained micaceous sand ac- cumulated to heights of about 12 ft above the canyon floor. Although the flood severely scoured the canyon in The Narrows, enough material remained in the channel to enable reconstruction of the pioneer highway without hauling in additional fill. The road embank- 118 FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO FIGURE 82 (above and facing page).—Site of the Loveland powerplant at mile 41.3. A, Before and B, after the flood. All but the poured-slab foundation and the bolted-down generators was destroyed. Bouldery flood gravel was about 20 in. thick. ment was simply rebladed from the valley-fill material to its preflood configuration and elevation. The flood emerged from The Narrows at the mouth of the canyon (fig. 85) with a calculated peak discharge of 31,200 ft3/s (Part A, table 3). NORTH FORK BIG THOMPSON RIVER Some of the heaviest rainfall of the July 31, 1976 storm fell on the North Fork just west of Glen Haven (pl. 2A). Flooding and its accompanying damage on the North Fork began near Glen Haven, where Devils Gulch poured substantial amounts of floodwater and debris into the North Fork. Below Glen Haven, the flood traveled down a sinuous, often narrow canyon to Drake—about 81/2 mi away and 1,170 ft lower. Near Drake, the discharge of the North Fork was almost 7 times greater than the previous maximum. (See Part A, table 3.) The canyon of the North Fork has long been a tourist attraction. Here and there were summer cabins, year- around homes, and daytime-picnic grounds; fortun- ately, the light population and a restriction on over- night camping held down the losses of life and proper- ty as compared with the main stem of the Big Thomp- son River. ABOVE GLEN HAVEN The small intermittent tributary in the upper part of Devils Gulch and the tributaries of West Creek, which enters Devils Gulch from the west, were responsible for most of the flooding in the Glen Haven area (pl. 2A). Devils Gulch and West Creek had calculated peak discharges of 2,810 ft3/s and 2,320 ft3/s, respectively; whereas the North Fork near the west end of the town had a calculated peak discharge of 890 ft3/s (Part A, table 3). GEOLOGIC AND GEOMORPHIC EFFECTS, BIG THOMPSON CANYON AREA 119 DEVILS GULCH Along the upper reaches of Devils Gulch, above mile 1.2, major sheet erosion and gullying occurred on the hillslopes and upland areas. Sheetwash from adjoining slopes was particularly widespread in the small upland meadows just north of the H bar G Ranch. Along this reach, the lower segments of most of the larger side gulches that join Devils Gulch were scoured to bedrock. A short segment of Devils Gulch about 1.5 mi above Glen Haven was also scoured to bedrock. Culverts were destroyed along the steep (11 percent) upper reach of Devils Gulch, and large gullies were scoured in their place. As the road was overtopped, scouring on one or both sides of the road undercut and destroyed the pavement. The destruction of the Devils Gulch Road severed access to Glen Haven and the North Fork from Estes Park. WEST CREEK A small tributary that enters West Creek from the south, about 1,100 ft west of Devils Gulch, received in- tense rainfall (pl. 2A). Floodwaters as high as 8.5 ft above present stream level removed as much as 6 ft of surficial material, scoured the channel to bedrock, and deposited a large gravelly fan at the mouth of the tributary. Lateral scour on the east-facing colluvial slope of the tributary triggered two small debris slides. Most of the drainage basin of West Creek upstream from this tributary, and most of Cow Creek were out- side the area of heaviest precipitation, although West Creek rose 5 ft above present stream level just 500 ft upstream from the tributary. Along Devils Gulch from mile 1.2 to Glen Haven (pl. 2A), the major effect of flooding changed from scour to deposition as the stream gradient decreases from about 11 percent to 2 percent and the valley bottom widens. In this reach, side slopes were slightly modified by sheetflooding and gullying. Small scoured tributaries flushed pebbly alluvium into the mainstream or deposited fans at the bases of slopes. Erosion was intense along unimproved roads. Gravelly sand accumulated to a thickness of about 3 ft where West Creek enters Devils Gulch. In overbank 120 FLOOD, JULY 31—AUGUST 1. 1976. BIG THOMPSON RIVER. COLORADO FIGURE 83.—Helicopter view of the Cedar Cove area near mile 39.6. Floodwater deeper than 20 ft above present stream level deposited a blanket of pebbly sand 3 or more ft thick. Boulders in the main channel were locally derived. Boulders on the high terrace remnant above the channel on far bank were deposited by an ancient flood. areas, sand was trapped by trees, shrubs, and grasses. Buildings, access bridges, and high ground also trapped as much as 3% ft of pebbly cobbly sand on their upstream sides. Many buildings were damaged or destroyed by flood- water and floating debris. Well-constructed masonry structures fared better than wooden ones, even though many of them lost doors and windows and were partly filled with sediment. Some more lightly constructed frame buildings were destroyed when their upstream sides caved in and their downstream walls were pushed out (fig. 86). On the southeast side of Glen Haven, runoff gullied the hillside, scoured the tributary gulch, and deposited a large sandy fan that spread into Devils Gulch and engulfed buildings and automobiles. WEST OF GLEN HAVEN Although as much as 12 in. of rainfall was recorded about 1 mi west of Glen Haven between the North Fork and Fox Creek, the storm caused only limited slope erosion'and minor damage to gravel roads within the vicinity of Glen Haven. Both Fox Creek and the North Fork overflowed their channels. Above their confluence, Fox Creek was as much as 5 ft above pre- sent stream level at mile 0.2; the North Fork was 2.5 ft above present stream level at mile 1.4 and was 3.7 ft above present stream level about one-tenth of a mile west of Glen Haven. GLEN HAVEN TO MILE 6.7, INCLUDING PIPER MEADOWS DRAINAGE From Glen Haven downstream to mile 6.7, the North Fork flows through a narrow, winding, steep-sided canyon walled by metamorphic rock. In some places the canyon is only 60 ft wide at river level. Consequent- ly, the flood rose as high as 12.5 ft above present stream level, and it severely eroded its channel and ma- jor portions of the road. Redeposited mainstream alluvium was mostly pebble to cobble gravel. FIGURE 84.—Flood damage at the mouth of Big Thompson Canyon, US Highway 34. At upper left of center is a short segment of the toppled overhead siphon, caught against a concrete abutment. The rest of the 110-t0n siphon was carried farther downstream. GEOLOGIC AND GEOMORPHIC EFFECTS, BIG THOMPSON CANYON AREA 121 122 FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO FIGURE 85.—View into The Narrows of Big Thompson Canyon taken from the canyon mouth the afternoon of August 2, 197 6, during heavy rainfall. US. Highway 34 truncated by scour. Adverse weather and low clouds seriously hampered rescue efforts within the canyon. Crest had fallen but river was still in flood. See also figures 63 and 84. At mile 7.3, the drainage from Piper Meadows enters the North Fork from the south at Glen Haven picnic ground. Although the basin is only 1.4 miz, this inter- mittent drainage produced by far the highest runoff of any tributary in the storm area (Part A, table 3). It had a calculated peak flow of 9,670 ft3/s—or about 30 per- cent of the maximum discharge recorded at the mouth of the Big Thompson Canyon during the flood. Despite the intense short-duration rainfall and rapid runoff in the Piper Meadows, degradation of the upland surface was relatively minor. Protected by dense native grass, the meadow area sustained little damage. Sheetflooding from the adjoining forested slopes washed sediment into the meadow and deposited about half an inch of sand. In the drainageways, where the gradient was about 7.5 per- cent, fast-flowing water as much as 3.5 ft deep and 25—30 ft wide flattened the vegetation. Where the sod was locally thin or absent, however, channels were scoured as much as 3.5 ft deep and 10 ft wide. The dirt road in Piper Meadows was gullied only to depths of a foot or two. Because the road follows the ridge crest, most of the drainage was away from rather than towards the road, and damage, therefore, was minimal. Below Piper Meadows the gradient of the tributary increases to about 12 percent, and the drainage is con- fined to a narrow gulch bottom. Along this reach flood- waters 8 ft deep cut a channel as much as 6 ft deep and 27 ft wide through alluvial and colluvial material, ex- posing bedrock. Lateral cutting below steep colluvial slopes triggered a few small debris slides. Rivulets down the steep side slopes scoured out as much as 2 ft of surficial material. About 0.6 mi above the North Fork, conditions in the ’Piper Meadows area changed markedly" as the drainage crossed from a granitic to a metamorphic ter- rane. The metamorphic terrane is steeper, and runoff GEOLOGIC AND GEOMORPHIC EFFECTS, BIG THOMPSON CANYON AREA was confined to a steep-sided channel cut in bedrock. Gaining energy, the flow abraded and plucked the steeply foliated rock. Just above the confluence with the North Fork, the water was as deep as 9.5 ft, and it scoured out much of the unconsolidated material from the 45-ft-wide bedrock-walled channel. Lateral cutting in colluvium triggered small debris slides. Although channel alluvium was intensively scoured, erosion did not expose bedrock in the bottom of the channel. At the confluence with the North Fork, the Piper Meadows floodwater, although charged with debris, contained little boulder-size material. On entering the North Fork it deposited a cobbly gravel bar that partly buried the picnic area downstream (fig. 104), and it scoured out the road on the opposite side of the canyon. MILE 6.7 T0 MILE 5.0 Leaving the confinement of the narrow canyon, the North Fork meanders 1.6 mi through a broad valley, 123 with a gradient of about 2 percent. Here, the flood damaged or destroyed roads, bridges, and buildings. The increased valley width and decreased gradient limited lateral erosion and caused extensive deposi- tion. Pebble to cobble gravel was deposited in the chan- nel, especially along insides of bends. Low terraces, especially between meander loops, were blanketed with pebbly sand, often in a braided pattern. Overbank deposits varied considerably in thickness, but com- monly they were thickest on rough ground. Buoyant debris and gravelly sandbars as thick as 3 ft were deposited where flooding was retarded by trees, fences, buildings, and road embankments. Gravel bars were derived chiefly from sources immediately upstream. At mile 5.3, the stream cut off a meander loop and filled the abandoned channel with sand and gravel. DUNRAVEN GLADE Tributaries entering the North Fork from the north and west between miles 6.7 and 5.0 were outside the FIGURE 86.—House in the lower end of Devils Gulch, near Glen Haven, demolished by hydraulic forces and impact from floating debris. The upstream side of the house was pushed in and the downstream side was pushed out. Note thick overbank sand and flood debris caught on trees and fences. 124 area of most intense rainfall but were flooded never- theless. In 2.5-mi-long Dunraven Glade, sheetflooding and gullying were confined mostly to the sparsely vegetated southwest-facing slope. These processes were intensified in and along the hillside road, where intense scour cut gullies as much as 5 ft deep and 20 ft wide into humic sandy surficial material. In the upper part of the tributary, sheetflooding deposited isolated patches of fine to coarse pebbly sand an inch or two thick in grassy areas. Downstream, a sandy veneer 0.5—2 in. thick was deposited in the valley bottom. The lower part of Dunraven Glade was only moderately gullied, but roads in the same area were extensively gullied and were washed out where culverts failed. A fan of pebbly sand was deposited at the confluence with the North Fork. MILLER FORK Miller Fork, the next north-side tributary below Dunraven Glade, lacked visible effects of heavy rain- fall throughout most of its length. Within about 1.5 mi of its confluence with the North Fork, slopes and roads were slightly gullied and small fans were deposited at the mouths of intermittent minor tributaries. On the other hand, Black Creek—the chief tributary of Miller Fork—was scoured along much of its length, thus il- lustrating the spottiness of the downpour. Black Creek flows into Miller Fork about half a mile above the North Fork and has a gradient as steep as 17 percent. Its calculated peak discharge was 1,990 ft3/s (Part A, table 3). In the upper reaches of Black Creek, runoff from the steep rocky slopes scoured out the valley fill and most of the road. A debris fan with many boulders a foot or so across was deposited at the mouth of Black Creek on top of an older larger fan that contained lichen-covered boulders as much as 4.4 ft in length. Downstream from the mouth of Black Creek, Miller Fork locally scoured its channel and deposited over- bank pebbly sand. Also, two large debris flows formed on the saturated southwest-facing slope at the colluvium-bedrock interface. At its confluence with the North Fork, Miller Fork had a calculated peak discharge of 2,060 ft3/s. Miller Fork crested after the North Fork, and it deposited a large sandy debris fan across the channel of the North Fork. OTHER TRIBUTARIES The small tributary gulch at mile 5.8 that drains the northwest flank of Crosier Mountain contributed sandy debris to the North Fork. Erosion in this steep tributary was restricted mostly to the lower half a mile where sheetflooding and deep gullying eroded the thick surficial mantle. FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO Storm effects along tributaries on the north side of North Fork diminished downstream from Miller Fork. Local sheetflooding and gullying eroded southwest- facing slopes, producing small amounts of pebbly sand. Runoff in the first tributary below Miller Fork deposited a veneer of sandy alluvium on a preexisting fan and cut a channel about 4 ft deep and 15 ft wide that undermined and toppled a small building on the fan. Many buildings on debris fans elsewhere in the Big Thompson storm area were similarly damaged or destroyed. DEBRIS AVALANCHES Just downstream from the first tributary below Miller Fork, near mile 5.2, two small avalanches broke loose about 120 ft above the North Fork, cascaded down the steep south-facing slope, and dumped bouldery debris into the stream. This debris deflected the river, causing it to scour its north bank and ag- grade its south bank. The larger boulders remained in the channel, but the finer debris was carried off by the North Fork. At mile 5.1, a much larger debris avalanche originated below craggy outcrops about 520 ft above the North Fork. Sliding down a 55-percent slope, it left a conspicuous narrow scar on the hillside (fig. 87 ). Part way down the slope the avalanche bifurcated around a small bedrock knoll. Debris dumped onto the road and into the North Fork ranged in size from sand to boulders larger than an automobile, but most of the finer material was carried away by the flood. Below mile 4, the flood plain of the North Fork broadens, and even though the gradient decreases to 3 percent, scour of the mainstream channel was heavy. Many large trees along the river survived, but large culverts were moved, and bridges and sections of the highway were destroyed. In some places the highway alinement became the new channel of the river. Lateral scour also triggered a few small debris slides in col- luvium above the flood plain. At bends in the canyon, gravel bars aggraded and displaced the channel. Locally derived cobbles and boulders and buoyant debris commonly accumulated on the upstream ends of the bars; pebbly sand ac- cumulated on the downstream ends. Overbank areas were covered with pebbly sand that increased in thickness where trees had trapped much buoyant debris. MILE 5.0 T0 DRAKE From mile 5.0 downstream to Drake damage was confined mostly to the main stream. Tributaries enter- ing the stream from the south above mile 1.6 were scoured by runoff, but those from the north below mile GEOLOGIC AND GEOMORPHIC EFFECTS, BIG THOMPSON CANYON AREA 125 FIGURE 87.—Scar left by debris avalanche that entered the North Fork at mile 5.1. The avalanche originated just below craggy outcrops at right about 520 ft above the North Fork. It cascaded down the 55-percent slope (foreshortened in this view) dumping large boulders and smaller debris onto the canyon floor. Rainfall here was about 7 in. 3.0 showed no evidence of erosion. Two small intermit- tent tributaries between miles 3.0 and 4.0 that drain grassy upland areas underlain by thick surficial mantles were appreciably eroded and aggraded. The lower part of the tributary at mile 3.7 was scoured, and a debris fan was deposited at its mouth. Smaller fans of pebbly to cobbly sand accumulated at the con- fluences of some of the side gulches along this tributary. The tributary at mile 2.8 was also scoured; sandy material partly filled the lower end of the chan- nel and built up in the area behind the highway em- bankment. The runoff that overtopped the embank- ment deeply incised the short section of streambed be- tween the highway and the North Fork. The severest damage to the North Fork was between miles 5.0 and 4.0 (fig. 88). There, the canyon floor nar- rows locally to a width of about 50 ft, and the gradient increases to about 4 percent. Severe scour destroyed most of the road and removed all but a few large trees. Mainstream channel alluvium was extensively re- worked and redeposited primarily at the bends in the canyon. Small pebbly gravel bars accumulated on the insides of bends; whereas large bouldery gravel bars with clasts as much as 2X2.6><4.8 ft accumulated on some of the outsides of bends. A large bouldery gravel bar was deposited as an island at the lower end of the reach where the channel straightens and widens. From mile 3 to Drake, the North Fork coincides with the Thompson Canyon fault zone. The gradient is less than 2 percent along this broad reach, and the flood- water spread out to a width of about 300—500 ft, blanketing the area with pebbly sand. Gravelly sand- bars accumulated to thicknesses of 3 ft or more behind trees, buildings, and other obstructions. Locally de- rived cobble gravel accumulated downstream from confined reaches. Scour was limited to minor lateral cutting along outer bends. 126 FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO FIGURE 88.—Remains of a medium-size automobile flattened and wrapped around a large boulder in the North Fork near mile 4.5. The US. Army Corps of Engineers removed 197 such wreckages, some less and some more severely damaged, along the Big Thompson and the North Fork. At the State fish hatchery, 0.6 mi upstream from Drake, dams, waterways, roads, and bridges were damaged or destroyed. Diversion systems, raceways, and ponds were filled with 3—4 ft of silty sand and peb- bly gravel. As much as 3 ft of pebbly sand was deposited on lawns (Ron Boyd, Colorado Division of Wildlife, oral commun., Dec. 9, 1976). Between the fish hatchery and Drake, at about mile 0.4, the valley of the North Fork is constricted by an old debris fan at the mouth of Bobcat Gulch (fig. 105). In this short segment floodwater destroyed two access bridges and scoured the fill from around their abutments. Scour along the outside of the bend of the constricted channel destroyed parts of the highway. Below the fan the valley widens again toward Drake where the gradient decreases to 1.2 percent. Along this reach, as much as 4 ft of sand and gravel was deposited (fig. 89). BIG THOMPSON RIVER FROM CANYON MOUTH TO CONFLUENCE WITH SOUTH PLATTE RIVER CHANNEL MODIFICATION During the July 31—August 1, 1976 flood the channel of the Big Thompson River shifted laterally in many places in conjunction with bank cutting and bar building. In some places, especially between the canyon mouth and Loveland, supplementary channels near the outer edges of the flood plain were temporari- ly occupied. There were, however, no permanent reloca- tions of the river, such as a new course across a meander neck, between the canyon mouth and the South Platte River. Bank cutting on outsides of curves, commonly associated with widening or downstream extension of point bars on insides of curves, took place at intervals determined in large part by the spacing of changes in GEOLOGIC AND GEOMORPHIC EFFECTS, BIG THOMPSON CANYON AREA 127 FIGURE 89,—Pickup truck partly buried by coarse sand deposited by the North Fork (in background) near its confluence with the Big Thompson. Note debris on cab of truck. in window of house on right. and on gravel bar in the background. Flow in this view was from left to right. channel direction. Minor bank cutting became nearly continuous as the channel pattern changed downstream from long nearly straight reaches and a few bends east of the canyon to many meander loops in the wider part of the valley approaching the South Platte River. Most commonly, where the extent of cut- ting or slumping could be determined or estimated, a bank width of 3—6 ft or less was affected. Deeper inva- sion of banks was seen in several places, however. Comparison of preflood (1971) and postflood aerial photography indicates that lateral scour cut at least 50—75 ft into the south bank between the powerplant near the canyon-mouth dam and an area beyond the first bridge below the dam. Houses on the south bank 10—12 ft above the river were undercut, and the bridge was destroyed. South-bank cutting was paralleled by deposition of a north-side point bar of sandy to bouldery gravel 100 ft wide and about 500 ft long. Other segments of the banks between the dam and the first hogback were cut back by amounts smaller than that in the area of the first bridge. At the first hogback, the river is between steep banks of hard bedrock or of artificial fill on which a narrow road had been constructed. Flood damage in this reach was restricted to destruction of parts of the road and underlying fill and removal of a steel bridge across the river (fig. 90). Southwest of Loveland in high banks 500—600 ft west of Taft Avenue, an estimated 20 ft of lateral ero- sion was evident (fig. 91). On the upstream side of a meander loop 500 ft west of this area, the river cut into an 8—10-ft bank of old pebble and cobble alluvium, redistributing some of this material on the point bar on the tip and downstream side of the meander. Here and elsewhere on point bars, the clearest evidence of move- ment of coarse sediment by the flood was partial burial of grass, cattails, or smaller weeds beneath cobble or pebble gravel and sand (fig. 92). Alluvium at channel edge was eroded back at least 20 ft in a 6—8-ft bank in the Riverview Campground about 2.5 mi downstream from the canyon mouth. In a pasture about 1 mi east of US. Highway 287, a high 128 FLOOD, JULY 31—AUGUST 1, 1976. BIG THOMPSON RIVER, COLORADO FIGURE 90.—View of damage to road and abutment of steel bridge torn out by Big Thompson River where it crosses first ridge east of Big Thompson Canyon. FIGURE 91.—Lateral cut, estimated at 20 ft, into south bank of the Big Thompson River west of Taft Avenue, southwest of Loveland. Abandoned gravel pit on the left was flooded. bank was eroded back far enough to destroy vehicle tracks 7 -8 ft wide near the bank edge. Comparable ero- sion appeared to have taken place in other areas as well, but we had no basis for making a specific estimate of the amount of bank removed. Bank conditions varied widely after the flood. Grass or weeds growing on some steep cut banks indicated that bank materials were not everywhere eroded by the recent flooding, even on the outer sides of channel curves. More often, however, especially in the western part of the valley, the preflood ba face and its vegetation were stripped away; along wooded banks, tree roots were extensively exposed. In low pastureland, where the turf was thick, many detached blocks of turf and soil slumped to the water’s edge, or sagged where the turf remained attached but where the underlying soil and fine-grained alluvium had been washed away. The undermining of turf 7—8 ft or more in width north of J ohnstown may be characteristic of unusual flooding. The more common narrower turf blocks suggest a flood magnitude no greater than that which, according to residents closer to the South Platte River, occurred “several times” in recent years. In general, bedrock along the channel banks was lit- tle affected by the flood. Some of the resistant ridge- forming sandstones in the hogback belt had only a faint stain line at high-water level. On the south side of the river less than 2 mi west of US. Highway I—25, GEOLOGIC AND GEOMORPHIC EFFECTS, BIG THOMPSON CANYON AREA 129 FIGURE 92.—Movement of small cobbles and pebbles on a point bar indicated by partial burial of cattails 2—3 ft tall; about 600 ft east of US. Highway 287 south of Loveland. however, floodwaters cut into soft shale banks below a sharply defined, nearly horizontal line that appeared to be about 5 ft above normal river level. A few partly detached blocks of weathered shale and vegetation still clung to the bank below the flood-cut line. Locally, banks were modified by water returning from overbank areas to the river channel by way of a low place in the edge of the bank. In such places the bank edge was eroded, and a crude fan of sediment was deposited on the bank face. OVERBANK DEPOSITION Deposition over the channel banks of the Big Thompson diminished rapidly downstream. Four ma- jor zones of overbank sedimentation between the canyon mouth and the confluence with the South Platte were recognized (pl. 2A, 23). Each zone graded into the next, but each had an average identity in terms of sediment grain size, maximum thickness, distribution, and sedimentary structure. Some of the sediment described in the following paragraphs was removed in cleanup operations after the flood. ZONE 1, CANYON MOUTH TO BIG THOMPSON SCHOOL The first sedimentation zone downstream from The Narrows (mile 37 to mile 34.3) crosses two valleys and two hogback ridges in about 2.5 mi. Within this zone, overbank sediment was dominantly fine to very fine, very micaceous sand. Maximum thicknesses were pro- bably as much as 6 ft in the western valley segment of the zone and approximately 4 ft near the east end of the zone. Thickness varied nonuniformly both downstream and between the channel edge and the outer edge of deposition. With few exceptions, the deposits were flatbedded and without obvious vertical gradation in grain size. Between the mouth of the canyon and the first hogback ridge, overbank flow and deposition took 130 place mainly on the north side of the river. Some current-swept areas near the riverbank retained only an inch or less of sand caught in turf. Sand deposits were thickest generally on the downstream or lee side of trees and debris piles and around houses. Where cur- rents moved freely between obstructions, the sand was thinner. The outer parts of lee deposits were cut away at two or more levels as the water surface dropped. As much as 4—5 ft of sand and some coarser material ac- cumulated around houses and as lee deposits at the first bend in the river below the dam. The greatest volume of sand, however, appeared to be immediately upstream from the hogback ridge. A right-angle change in channel direction and constriction of flow where the river entered the narrow passage through the ridge, as well as diversion of part of the flow across the flood plain toward Sulzer Gulch, presumably reduced the rate of flow and forced deposition of a ma- jor part of the sand load. After the flood, low channels FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO between terraced mounds of sand as much as 3—6 ft thick made a ragged landscape that probably bore lit- tle relation to the preflood surface (fig. 93). Although micaceous fine sand was by far the most abundantly deposited material, both coarser and finer sediments were left in parts of the overbank area. Cob- bles, pebbles, and coarse sand from the channel moved over the low banks of the first river bend below the dam, and silt settled from water that was trapped in houses there and farther downstream. In a roughly triangular area at the mouth of Sulzer Gulch, near the hogback on the north side of the Big Thompson River, slightly silty very fine sand settled out of water that presumably was ponded by the large volume of sand deposited immediately upstream from the ridge. In this area, desiccation cracks formed irregular blocks 5—8 ft in maximum dimension (fig. 94). Closer to the main channel of the river, silt made up less than 10 per- cent (estimated) of most of the sandy alluvium. FIGURE 93.—Flood-deposited sand near confluence of Sulzer Gulch and Big Thompson River showing two terraces out below highest level of deposition. GEOLOGIC AND GEOMORPHIC EFFECTS, BIG THOMPSON CANYON AREA 131 FIGURE 94.—Desiccation cracks in slightly silty very fine sand trapped at mouth of Sulzer Gulch, west of first ridge east of mountain front. Large blocks are 5-8 ft in maximum dimension. The floodwaters also deposited in this area large amounts of debris from above the canyon-mouth dam—trees, bridge timbers, parts of houses, highway markers, and other buoyant items. Several houses along this reach of the river were partly or totally destroyed by the force of the water or impact of debris. Along the edge of the flood plain, some trees and large masses of turf were torn out and pines were stripped of bark on the upstream side to heights of 9 or 10 ft above ground level (fig. 95). Bark from pines within this zone or farther upstream was found as far east as the J ohnstown area, more than 20 mi downstream. The narrow passage of the river through the first hogback is a small canyon that contains almost no overbank area capable of accumulating flood deposits. Downstream from the first ridge, the river follows a slightly sinuous course through a narrow flood plain in a southward extension of Green Ridge Glade. Most of the overbank sediment in this area was fine to very fine micaceous sand, thinly deposited in open areas but nearly 4 ft thick in the lee of large debris piles in the cottonwood groves north of the Highway 34 bridge (fig. 96). Unobstructed flood currents terraced the thick deposits of sand at one or two levels and left grass exposed in open areas, especially near bank edges. Here, as in other groves farther downstream, massive amounts of broken trees and other buoyant debris were filtered from the flood by standing trees that were able to resist the impact of the flood load. In some places the resulting debris dams provided protec- tion to houses built within several of the groves, but, elsewhere, houses directly exposed to impact were partly destroyed—some beyond reconstruction (fig. 97). Some buildings were rotated on their foundations. High-velocity flat bedding was the only sedimentary structure observed in the sand, except very local slump structures in beds that were laid down against border- ing slopes at the outer edges of the flood plain. Sand 132 FLOOD, JULY 31-AUGUST 1, 1976. BIG THOMPSON RIVER, COLORADO FIGURE 95.—Upstream side of ponderosa pines stripped of bark by impact of flood debris, between dam at mouth of Big Thompson Canyon and first ridge east of canyon; note also, destruction of flood-plain turf. FIGURE 96.—Current-terraced deposit of sand. 3—4 ft thick. on downstream side of debris pile in cottonwood grove about half a mile west of Big Thompson School. GEOLOGIC AND GEOMORPHIC EFFECTS, BIG THOMPSON CANYON AREA FIGURE 97.—Condemned damaged house and debris pile in cottonwood grove about half a mile west of Big Thompson School. beds plastered against a steep curved border slope west of the river near US. Highway 34 extended at least 15 ft above late-August river level; close to the channel edge, 2 or 3 ft above water level, only a thin layer of sand was laid down. Sediment both coarser and finer than the dominant fine sand was laid down in small quantities in over- bank areas in the valley south of Green Ridge Glade. Dark clayey silt collected in a ponded area between the north side of the US. Highway 34 embankment and the ridge slope in the gap through the second hogback. Cobbles, pebbles, and coarse sand were noted in many places along the glade at or near bank edge. Channel edges here are generally very low banks or are grada- tional into the flood plain without distinct banks. Movement of coarse material from channel floor or cut banks to flood plain apparently had been accomplished easily in many places. How much of the coarse sedi- ment reached the flood plain in the 1976 flood and how much in earlier floods of smaller magnitude is not clear, but recent movement of such material on the flood plain was indicated by crudely imbricated deposits of pebbles and cobbles over grass and by a cobble resting on a small transported tree. ZONE 2, EAST OF BIG THOMPSON SCHOOL TO LOVELAND In this zone (mile 34.3 to mile 25.0), silt was a major component of overbank sedimentation and increased downstream in proportion to sand. Sand, ranging from coarse to very fine, was still dominant near river- banks. In bordering fields and cottonwood groves, deposits graded outward from sand through silty sand, sandy silt, and in some places silt or clayey silt. All overbank sediments were highly micaceous. Maximum thicknesses, usually close to the river, were nearly everywhere less than 2 ft. Changes in thickness across any one area were usually nonuniform. As in zone 1, thicker than average deposits accumulated on the downstream sides of trees and brush (fig. 98). Terraced mounds of sand not obviously trapped by vegetation were left isolated in several fields. In general, sand 134 um- ., 9. FIGURE 98.——Lee deposits of flood-borne sand on down-current side of bushy weed (3—4 ft original height) east of Glade Road (sec. 7, T. 5 N., FLOOD, JULY 31-AUGUST 1, 1976. BIG THOMPSON RIVER, COLORADO R. 69 W.). deposits were flatbedded, but in a few places minor slump bedding developed or final draining of water left a thin coating of rippled sand. Modification of flood effects by human use of the flood plain increased downstream toward Loveland. In fields southwest of the city, maturing corn was broken off or overridden by broken trees and other debris that floated across the fields or piled up in ragged wedges well within the stands of corn (fig. 99). Water flowing through the fields downstream from the debris piles left only a thin deposit of silt. This type of crop damage occurred more often in the heavily farmed segment of the valley east of US. Highway I—25 than in the area upstream from Loveland, but the reduced amount of both debris and water downstream caused less destruction, in most cases, to the invaded fields. Close to Loveland, large gravel pits, both active and abandoned, gradually displace agricultural use of the land. Where floodwater broke into these pits and was temporarily retained, clay and silt settled out. Pits not normally containing water were floored with mud- cracked sediment after the floodwater drained away. In the urbanized area south of Loveland, artificially high banks armored with riprap prevented overbank flow in some places but were overtopped or were ab- sent in others. Where the river passes through the Larimer County Fairground, very little bank erosion occurred because of extensive riprapping, but nearly all parts of the grounds were flooded and blanketed with fine sediment. A dealer in used auto parts, east of US. Highway 287, reported thin silt throughout his car lot. In an unprotected low part of the lot, 18 in. or more of sand partly buried an auto body; in a slightly higher area closer to the river, only patchy sand and silt settled around bent-over grass that still indicated flow patterns two and a half weeks after the flood. GEOLOGIC AND GEOMORPHIC EFFECTS, BIG THOMPSON CANYON AREA 135 FIGURE 99.—Flood damage in cornfield southwest of Loveland. Debris-laden water, moving from left to right, broke cornstalks in part of the field and drove wedges of broken trees and other debris into corn that remained standing. Approximate scale: 1 in.=250 ft. ZONE 3: SOUTHEAST 0F LOVELAND Through most of the zone, the proportion of silt to To LARIMER‘WELD COUNTY‘LINE ROAD sand increased downstream. All examined deposits Thickness of overbank silt and sand in this zone were highly micaceous, except as noted in the follow- (mile 25 to mile 15.7) generally decreased downstream ing paragraph. from a maximum of about 1 ft to less than 6 in. About 1.5mieast of US. Highway I—25, coarse sand 136 increased abruptly in overbank deposits. For nearly a mile along the south side of the river, coarse to medium sand was concentrated near the edge of the overbank area and in shallow linear depressions paralleling the bank. Thin deposits of finer sand or sandy silt between the depressions graded to silt away from the river or to clayey silt in old meander scars where floodwater was ponded. The percentage of mica in overbank sediment was markedly lower than the amounts observed both upstream and downstream from this area. These local changes in overbank sediment indicated local sources in the stream channel or in fresh cut bank alluvium. In this area, also, on the north side of the river, a downstream splay of large pebbles and coarse sand across very low bottomland was traced 40—50 ft direct- ly back to a bank cut containing old gravel near water level. Some of the variation in amount and texture of overbank sand in zone 2 also may be due to local derivation of sediment, but the evidence is less con- clusive there than in zone 3. ZONE 4, LARIMER-WELD COUNTY—LINE ROAD TO CONFLUENCE WITH SOUTH PLATTE RIVER Overbank sediment in zone 4 (mile 15.7 to mile 0) was predominantly silt. Fine sand and patches of coarser sand were deposited locally near bank edges. Overbank deposits were generally micaceous. Along some very low banks, pebble and sand splays or coarse to medium sand tongues extended over bottomland. Thickness of overbank sediment was generally less than 6 in. and commonly less than 1 in. On inundated pasture land, areas apparently free of any flood sedi- ment occurred within the larger area of flood-deposited silt. Distribution of the very thin silt deposits seemed to have been controlled by nearly imperceptible varia- tions in the surface. Outer edges of silting showed sharply against some sloping surfaces but were in- distinct on many flatter surfaces. In crop areas it was sometimes difficult to distinguish flood silt from silt redistributed by irrigation of the fields. In parts of zone 4, overbank deposits were 1eSs read- ily differentiated from channel-edge sediments than in much of the upstream area. Abundant meander scars locally merge channel and adjoining bottomland in a bankless quagmire of boggy mud churned by cattle. Floodwaters added silt and sand to these areas of con- fusion. As the thickness of overbank deposits decreased downstream, recognition of the outer limits of such deposition became more difficult. To assist in locating the probable limits, an effort was made to estimate the height to which the water level rose. These estimates in FLOOD, JULY 31—AUGUST 1. 1976, BIG THOMPSON RIVER, COLORADO zone 4 were made in part on sediment distribution and in part on the basis of grass caught on trees or fence wires and apparent high-water marks on trees and fence posts. Some flood sediment was found on ground higher than such markings, or water in higher fields was reported by farmers. Some of the so-called high- water markers, therefore, were interpreted as possible pause levels of receding floodwater. Our best control- led estimates in zone 4 indicated rises of at least 8 ft (above late-August water levels) in the upstream loca- tions and about half of that amount where the Big Thompson River flows in a dredged channel between confining banks about a mile from the South Platte River. Residents along the South Platte River about 1.5 mi below the Big Thompson River confluence reported a water rise of between 1 and 2 ft at most. Although freshly broken or uprooted trees were noted on the flood plains in zone 4, a large part of the debris appeared to be tree trunks and branches from previous overflow or from causes unrelated to flooding. Much of this material was redistributed by the 1976 flood. US. Army Corps of Engineers personnel, work- ing with cleanup crews, stated that large amounts of preflood debris were being removed with that of the re- cent flood. DAMAGE CAUSED BY GEOLOGIC PROCESSES DURING F LOOD-PRODUCING STORMS By JAMES M. SOULE2 GEOLOGIC HAZARDS3 Damaging processes that were active in geologic- hazard areas above mainstream flood levels during the 1976 Big Thompson flood may be grouped under (1) water transport of soil, rocks, and vegetation debris and (2) mass wasting of slope materials. Water transport processes were sheet erosion and deposition of surficial materials, downcutting of drainageways, mobilization of large debris (including vegetation and manmade structures) on hillslopes, and deposition of debris on debris fans or movement of debris across debris fans and into major stream courses. Mass wasting produced landslides, including slumps, 2Colorado Geological Survey, 1313 Sherman Street, Den-vet, Colorado 80203. 3In this section, “geologic hazards" refers mainly, but not exclusively, to such hazards above the legally defined 100-year-flood plain of the Big Thompson River. According to Col- orado statute 106—7—103 (8) C.R.S., waterflooding is not considered to be a geologic hazard. For a discussion of the State of Colorado‘s involvement in geologic-hazard area identifica- tion and legel definitions of geologic hazards, see Rogers and others (1974). GEOLOGIC AND GEOMORPHIC EFFECTS, BIG THOMPSON CANYON AREA 137 1 05°25’30” 105°26 ’30” CONTOUR INTERVAL 40 FEET EXPLANATION Aag Debris fans, A—a indicates activity on the /\ -.~. debris fan during the 1976 storm; g / Isa Landslide areas (pebble to cobble) and b (boulder) indicate the predominantsize of material in the debris fans Rockfall areas 9 Direction of surface flow Unstable or potentially unstable slopes Sheet-erosion areas A ro' teli it of 1976 flood 4e+ Dom mm; - 5:;“;“:;;;;m Riv; °“ W Landslide Site of peak discharge measurement FIGURE 100.—Geologic hazards in the Glen Comfort area, between mile 53.4 and mile 51.9 on the Big Thompson River, are typical of much of the Big Thompson area (modified from Soule and others. 1976). Base from U.S. Geological Survey 1:24,000 Glen Haven, 1962. 138 rockfalls, rockslides, debris avalanches, and debris slides. The most common damages to structures were caused by impact by moving vegetation and earth- materials debris, failure or erosion of slopes or of road- ways and other earthfill structures, and partial to com- plete burial of buildings by silt- to boulder-size col- luvium or alluvium or by vegetation debris. The rates at which these processes operated apparently varied from nearly imperceptibly slow to exceedingly rapid. Distribution of geologic hazard areas along part of the Big Thompson River is illustrated in figure 100. During the 1976 Big Thompson storm and flood, the disastrous effects of geologic processes on life and property above the lOO-year flood level were mainly in areas of maximum rainfall, highest rates of rainfall, or greatest runoff. Geologic processes that could be activated by future storms have been identified in many other places throughout the Big Thompson area. Heavy damage to inhabited areas can happen, as it did in 197 6, in heavy rainfall areas where runoff is chan- neled into steep tributaries and mainstream courses. Flash flooding in such places was so sudden and of such magnitude that little, if any, warning of the im- pending danger to residents was possible. In areas of light rainfall during the July 31, 197 6 storm, much of the damage in geologic-hazard areas close to the flood level of the Big Thompson and the North Fork was caused by the swollen waters of the main streams, which picked up and transported much of their bed materials and undercut adjacent slopes. In the examples discussed subsequently, much severe damage was caused by geologic processes acting above the legally defined 100-year flood plain. Not unexpectedly, many such areas have been pre- ferred building sites in the past and presumably will continue to be under pressure for part-year or full- time recreational-residential development in the future. Many of the geologic hazards in these areas are the results of land use that was constrained by lack of practical alternative building sites. Early development of canyon bottoms and a few other gently sloping areas came about because of the attractiveness of streamside building lots and the difficulties associated with con- struction, water availability, and the accessibility of steeper valley sides. Because relatively large, esthetically pleasing building sites were available at some stream confluences and because these same areas provided easy access slightly above the mainstream flood plains, they have become the sites of most small communities in the canyon area. In recent years limited development has taken place in a few higher areas away from streams. In developed high areas that have gentle to moderate (5—15 percent) FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO slopes, erosion and redeposition of granitic grus and soil caused damage during the storm of 1976. In the relatively steep (>30 percent slope) parts of these high areas, places susceptible to mass slope movements, such as slumps and debris slides, are common. Slump- ing seems to be more common on predominantly north facing slopes, owing to the presence of relatively thick colluvial deposits. Predominantly south facing slopes, where the veneer of debris is thinner, are susceptible to debris sliding and debris avalanching. Rockfalls usual- ly occur below cliffs. Although few of these mass movements are known to have caused major damage during the 1976 storm, the potential for future loss of life and property is great if susceptible areas are developed for residences. DAMAGE IN GEOLOGIC HAZARD AREAS Damage related to geologic hazards was most widespread where rainfall was very heavy, but severe damage in downstream areas of lighter rainfall resulted from erosion and deposition by the trunk streams, which carried abundant upstream runoff and debris. Damage in several types of geologic-hazard areas affected by the storm and flood, and the poten- tial for similar damage in the future, are discussed in the following paragraphs. DAMAGE ON DEBRIS FANS Because of their number, total area, density of residential development, and potential for damage dur- ing heavy regional rainstorms or intense local thunderstorms, debris fans are the sites of serious geologic hazards in the Big Thompson drainage basin. A debris fan or a related but less clearly defined deposit of stream-borne rock debris exists at virtually every stream confluence in the Big Thompson area, in- cluding those of small ephemeral streams. Damage may result when the rising mainstream overruns the fan or cuts part of the fan away, or when flash flooding of the tributary itself mobilizes debris on the fan (fig. 101). The amount and type of tributary damage may be influenced by the size, slope aspect, and nature of the surface materials of the tributary drainage basin and by the gradient of the tributary drainage. Figures 102 and 103 illustrate typical heavy damage in the 1976 storm to structures on relatively small debris fans. Slope aspect and composition of bedrock within a drainage sometimes influence the damage that may oc- cur on a debris fan. In small steep drainages where south-facing slopes predominate, large boulders and GEOLOGIC AND GEOMORPHIC EFFECTS, BIG THOMPSON CANYON AREA nearly unweathered rock debris usually constitute the bulk of the slope material. Where north-facing slopes predominate, vegetation commonly makes up much of the debris. This mixed debris can greatly affect damage inasmuch as large rocks and pieces of trees or other vegetation can block the drainage channels and thus locally concentrate debris and flooding. To predict the resulting damage on the debris fan, therefore, is difficult or impossible. Owing to vegeta- tion cover and greater soil moisture, predominantly north facing slopes tend to have deeper soil and less coarse weathering products than south-facing slopes. Surficial characteristics are similar where gentle slopes are composed primarily of granitic grus or fractured and weathered metamorphic rocks. Debris fans heading in these areas tend to consist of pebbly to cob- bly, as opposed to bouldery, material (fig. 104). Struc- tures built on such fans are perhaps unlikely to be damaged by impact from very coarse debris, but they are not less vulnerable to water damage than struc- tures on coarse-debris fans. Extensive flooding and spreading of bouldery debris on debris fans by tributary streams have occurred in the Big Thompson area in the past and could be a hazard in the future, although this did not happen in 1976. A conspicuous recent example of boulder flooding is at the confluence of Bobcat Gulch with the North Fork Big Thompson River (mile 0.5). As in- dicated in figure 105, the moderately large debris fan at this locality is strewn with little-weathered boulder debris that appears to be no more than a few hundred years old and may be considerably younger. The course of the North Fork seems to have been deflected toward its present location by one or more episodes of boulder- debris deposition. Although moderate erosion and flooding of the Bobcat Gulch debris fan during the 1976 flood was caused by the North Fork and not by the tributary drainage, the area clearly has potential for a catastrophic boulder flood, derived from the tributary. Consequently, Bobcat Gulch and similar areas are considered to be hazardous for houses and other structures. DAMAGE ON VALLEY SLOPES AND IN TRIBUTARY DRAINAGES Erosion and deposition of fine-grained surficial materials caused significant damage on valley slopes. Where heavy rain fell on steep, openly wooded slopes without protective ground cover of grass or other vegetation, sheet flooding eroded large amounts of loose surface material (p. 100). Sheet erosion also oc- curred on many gentle slopes where virtually no drainage net existed before the flood and where grass 139 cover was sparse. In broad areas sheet erosion re- moved as much as 6 in. of material. In several resi- dential subdivisions above flooding streams, exten- sive damage was caused by deposition of sediment on building lots (fig. 106), inside buildings, and on roads by erosion of unpaved roads and drainage control structures. Gullying was especially troublesome in colluvial and residual materials derived from granitic rocks (fig. 107). In some places the breaking of grass cover by a wheel track had provided shallow channels that became gullies several feet deep (p. 100, 120). In many parts of the Big Thompson drainage basin, sheet erosion and gullying are major potential prob- lems for future residential developments on high ground above stream flood levels, especially where natural drainage is extensively modified by cuts and fills for roads and buildings or where sparse natural vegetation is greatly disturbed. DAMAGE IN LANDSLIDE AREAS Many places in the Big Thompson area are suscepti- ble to slope failure, and many slope failures took place during the July 31, 1976 storm. Fortunately, only a few caused serious damage, largely because most of the affected areas lacked residential development. In some places, small landslides partly buried roads. Small slumps from banks of deepened tributaries caued minor damage and diversion of floodwater. An existing landslide that was considerably en- larged by the 1976 flood caused the loss of a house and its access road (fig. 108). Several other canyon areas contain potential landslides that could be released in future storms and floods (Soule and‘others, 1976). Houses have been built in or downslope from some of these places. The several debris avalanches near miles 5.2-5.4 of North Fork Big Thompson River (fig. 109) caused no major damage, but similar surface conditions elsewhere in the Big Thompson area suggest vulnerability in places that were not damaged by the 1976 storm. Little apparent damage was caused by rockfalls in the Big Thompson area during the 1976 storm and flood, although moderate to extreme rockfall hazards exist on some of the steep canyon-side slopes. Minimal rockfall damage was due to two factors: (1) rockfalls in this area apparently are less commonly associated with storm precipitation and flooding than with winter and spring freeze-thaw cycles and (2) most rockfall hazard areas, because of their steep slopes and diffi- culty of access, have not been developed as building sites. FIGURE 101 (above and facing pageL—Aerial views of Noels Draw, mile 52.7 on the Big Thompson River, before and after the 1976 flood. A, Preflood view; B, postflood view, August 3, 1976. The bedload of Noels Draw had been transported and removed from the draw. This drainageway has a peak discharge of 6,910 fth CONCLUSIONS The Big Thompson storm and flood of 1976 were ex- ceptional but not unique events. Comparable storms and floods have happened in the Front Range area in the past and probably will happen in the future. The flash flood environment developed when very moist, conditionally unstable air, carried into the Front Range by strong easterly circulation, was lifted orographically, triggering the explosive growth of very FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO during the 1976 flood. (See Part A, table 3.) All structures in or near the streambed (outlined) have been destroyed, and the debris fan has been partly eroded away. The bridge and house that were on the fan before the flood are gone. Approximate scale: 1 in.=200 ft. large thunderstorms. Weak westerly wind aloft retard- ed the normal drift of the thunderstorms as moist air continued to flow into the system from the southeast at lower levels of the atmosphere, thus localizing ex- ceptional amounts of rainfall. Unprecedented unit-area discharges from small tributary basins of the Big Thompson River resulted from the high intensity of the downpour and the steepness of the terrain. Feeding into the main stem, the runoff from these tributaries caused a flood in the GEOLOGIC AND GEOMORPHIC EFFECTS, BIG THOMPSON CANYON AREA 141 canyon 4 times larger than any previously measured at the canyon mouth. The great destructive energy of the flood and the ability of the flood to bring about geologic change were caused by the steep gradients and, hence, the high current velocities. A pattern of hydrogeologic responses to high-energy discharge emerges from this study of a torrential mountain flood. Many of these responses seem self evi- dent, but they are applicable to analogous settings elsewhere, and their reiteration might help foster an awareness of similar hazards in other areas that have not experienced a disastrous flood. Following a heavy thunderstorm, a mountain torrent might increase its discharge and competence by several orders of magnitude, through a combination of meteorologic and physiographic circumstances. In the Big Thompson Canyon area, an extreme increase in discharge and velocity drastically raised the com- petence of the stream and, hence, its capacity to erode and transport material—in short, its ability to cause geomorphic change, damage property, and endanger life. Along the tributaries, geologic effects were con- fined largely to the zone of intense downpour, chiefly to places within the 6-in. isohyet and, foremost, within the 10-in. isohyet. But along the main stem, effects ex- tended to the confluence with the South Platte River. Under conditions of torrential discharge: - The main channel tended to be scoured throughout its length. 142 FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO House esseng‘ My un . _ 1. ‘ 2.? FIGURE 102.—Aerial views of debris fans, mile 51.1 to mile 50.9 on the Big Thompson River, before and after 1976 flood. A, Preflood view; B, postflood view, August 3, 1976. Note changes in the small debris fans and their associated tributary drainages (outlined); partial destruction of the house shown in figure 103 resulted. A large amount of boulder and vegetation debris is present on the predominant- ly south facing slopes in these tributary drainages. Approximate scale: 1 in.=200 ft. GEOLOGIC AND GEOMORPHIC EFFECTS, BIG THOMPSON CANYON AREA 143 FIGURE 103.—-House on debris fan at mile 51.1, Big Thompson River, damaged by boulder debris. Part of the building has been crushed. The house was pushed about 4 ft off its foundation. The mobile home to the right and another home out of view to the left (fig. 102) were virtually undamaged. FIGURE 104.—Deposit of sand- to cobble-size material on a debris fan near mile 7.3, North Fork Big Thompson River. Deposit is about 4 ft thick. The man is standing on a picnic table that was engulfed by the deposit. 144 FLOOD, JULY 31—AUGUST 1. 1976, BIG THOMPSON RIVER, COLORADO FIGURE 105.—Postflood aerial view of Bobcat Gulch debris fan, mile 0.5 on the North Fork Big Thompson River, August 3, 1976. The many fresh boulders on this debris fan indicate a relatively recent flood, although the water of the 1976 flood was deflected around this debris deposit. Approximate scale: 1 in. =200 ft. 0 The flood surface was not planar. Owing to centri- fugal flow, it tended to be superelevated at the outsides of bends, often several feet higher than the water surface on the opposite bank; struc- tures at the outsides of bends, therefore, were especially vulnerable to the effects of flooding. 0 Places opposite flooding tributaries were vulner- able to flooding by the deflected main stream or by the tributary itself, pushing across the mainstream channel. Scour predominated at the outsides of bends. Scour increased at channel constrictions; deposi- tion increased just upstream from constrictions. Scour increased greatly with an increase of gra- dient and diminished correspondingly with a decrease. Scour undercut colluvial slopes high above the peak elevation of flooding and thereby released landslides that endangered or destroyed struc- tures safe from ordinary flooding. GEOLOGIC AND GEOMORPHIC EFFECTS, BIG THOMPSON CANYON AREA 145 FIGURE 106.—Building lot covered by sediment, in a subdivision where colluvium derived from grus was eroded and redeposited. There was little preflood indication of the potential problem. Meadowdale area, near the divide between the Big Thompson and St. Vrain drainages. . Artificial fills in the floodway were very vulnerable to scour. . Scour was minimal in overbank areas. . The insides of bends were loci of deposition, often of bouldery point bars. . Deposition was enhanced by reduced gradients, sharp bends, widened places along the channel, and overbank flow. Reduced gradients and widened channels usually coincided. - Vegetation—trees, shrubs, log jams, and even grasses—and other obstructions were natural sediment traps in overbank areas. Dunelike bars commonly accumulated downstream from such obstructions. . Impacts from heavy buoyant debris, such as logs and timbers, locally caused more damage to structures than was caused by the floodwater itself or saltating boulders. In sheetflooded areas: - Scour was inhibited by thick mats of vegetation, especially grasses. - Scour was enhanced Where the ground cover was thin or discontinuous, as in a pine forest, or where the cover was broken, as along an unim- proved road. At debris fans: . Although many debris fans stood high above main- stem flooding, they contained inherent hazards because they are and were subject to episodic tributary flash floods; they in fact are formed from repeated increments of flood debris. FIGURE 107.—Gully erosion in fine-grained, relatively thick collu- vium or residuum derived from granitic rocks. Gully is about 4 ft deep in the foreground. Near mile 54.4 on the Big Thompson River. . Any part of a fan is vulnerable to flash flooding or debris flowage inasmuch as the flood channel on the fan tends to be blocked and diverted repeatedly by its own debris at various times during the history of the fan. . Only a random part of a given fan was flooded dur- ing the storm or is likely to be flooded during a future storm. SELECTED REFERENCES Balog, J. D., 1977, Big Thompson River tributaries: geomorphic ac- tivity and its controlling factors during the 1976 flood: Univ. Colorado, unpub. M.S. thesis, 81 p. FLOOD, JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO Braddock, W. A., Calvert, R. H., Gawarecki, S. J ., and Nutalaya, Prinya, 1970, Geologic map of the Masonville quadrangle, Larimer County, Colorado: U.S. Geol. Survey Geol. Map GQ—832. Braddock, W. A., Nutalaya, Prinya, Gawarecki, S. J ., and Curtin, G. C., 1970, Geologic map of the Drake quadrangle, Larimer County, Colorado: U.S. Geol. Survey Geol. Map GQ—829. Colton, R. B., 1978, Geologic map of the Boulder—Fort Collins— Greeley area: U.S. Geol. Survey Misc. Geol. Inv. Map I—855—G. Crosby, E. J ., 1978, Landforms in the Boulder-Fort Collins-Greeley area, Front Range Urban Corridor, Colorado: US. Geol. Survey Misc. Geol. Inv. Map I—855-H. Dennis, A. S., Schleusener, R. A., Hirsch, J. H., and Koscielski, A., 1973, Meteorology of the Black Hills flood of 1972: South Dakota School Mines and Technology, Inst. Atmospheric Sci. Rept. 73—4, 41 p. Ducret, G. L., Jr., and Hansen, W. R., 1973, Storm of May 5—6, 1973, in the Denver Metro area—Frequency and effects: Urban Drainage and Flood Control District, Denver, 0010., 20 p. Erbe, N. A., and Flores, D. T., 1957, Iowa Drainage Laws (annotJ: Iowa Highway Research Board Bull. 6, 870 p. Engeln, O. D., von, 1949, Geomorphology, systematic and regional: New York, The MacMillan Co., 655 p. Follansbee, Robert, and Sawyer, L. R., 1948, Floods in Colorado: US. Geol. Survey Water-Supply Paper 997, 151 p. Gary, Margaret, McAfee, Robert, J r., and Wolf, Carol L., eds., 1972, Glossary of Geology: American Geological Institute, 805 p., appendix A—l, A—52. Grozier, R. U., McCain, J. F., Lany, L. F., and Merriman, D. C., 1976, The Big Thompson River flood of July 31—August 1, 1976, Larimer County, Colorado: Colorado Water Conservation Board, Flood Inf. Rept., Denver, Colorado, 78 p. Hansen, W. R., 1973, Effects of the May 5—6, 1973, storm in the Greater Denver Area, Colorado: US. Geol. Survey Circ. 689, 20 p. Hansen, W. R., Chronic, John, and Matelock, John, 1978, Clima- tography of the Front Range Urban Corridor and vicinity, Col- orado: U.S. Geol. Survey Prof. Paper 1019, 59 p. Jenkins, C. T., 1961, Floods at Boulder, Colorado: US. Geol. Survey Hydrologic Inv. Atlas HA—41. Langbein, W. B., and Iseri, K. T., 1960, Manual of Hydrology—- General introduction and hydrologic definitions: U.S. Geol. Survey Water—Supply Paper 1541-A, 29 p. Maddox, R. A., Caracena, F., Hoxit, L. R., and Chappell, C. F., 1977, Meteorological aspects of the Big Thompson flash'flood of 31 July 1976: Natl. Oceanic and Atmospheric Adm. En- vironmental Research Lab. Tech. Rept. ERL 388—APCL41. Marr, J. W., 1964, The vegetation of the Boulder Area, in H. G. Rodeck, ed., Natural history of the Boulder area: Univ. Col- orado Mus., Leaflet no. 13, p. 34-42. GEOLOGIC AND GEOMORPHIC EFFECTS, BIG THOMPSON CANYON AREA FIGURE 108.—Landslide in Glen Comfort area, mile 52.4 on the Big Thompson River. This landslide was initiated, presumably, before the 1976 flood when a roadcut was made at the base of the slope. During the flood, the road and the house that it served were eroded away and destroyed. Additional erosion into the slope by the Big Thompson River caused further failure of the landslide. Note also the ero- sion under the house on the left. _ 1967, Ecosystems of the east slope of the Front Range in Colorado: Univ. Colorado studies, Series in biology, no. 8, 124 p. Matthai, H. F., 1969, Floods of June 1965 in South Platte River basin, Colorado: U.S. Geol. Survey Water-Supply Paper 1850—B, 64 p. McCain, J. F., and Hotchkiss, W. R.. 1975a, Map showing flood- prone areas, Boulder—Fort Collins—Greeley area, Front Range Urban Corridor, Colorado: U.S. Geol. Survey Misc. Inv. Map I—855—E. [Has extensive flood-data bibliography] 1975b, Map showing flood-prone areas, Greater Denver Area, Front Range Urban Corridor, Colorado: U.S. Geol. Survey Misc. Geol. Inv. Map I—856—D. [Has extensive flood- data bibliography] 1975c, Map showing flood-prone areas, Colorado Springs- Castle Rock area, Front Range Urban Corridor, Colorado: U.S. Geol. Survey Misc. Geol. Inv. Map I—857—C. [Has extensive flood-data bibliography.] McCain, J. F., and Jarrett, R. D., 1976, Manual for estimating flood characteristics of natural-flow streams in Colorado: Colorado Water Conserv. Board Tech. Manual No. 1, 68 p. National Oceanic and Atmospheric Administration, 1976. Big Thompson flash flood of July 31—August 1, 1976: Rockville, Maryland, Natural Disaster Survey Rept. 76—1, 41 p. Nilsen, T. H., 1972, Preliminary photointerpretation map of land- slide and other surficial deposits of the Mt. Hamilton quadrangle and parts of the Mt. Boardman and San Jose quadrangles, Alameda and Santa Clara Counties, California: U.S. Geol. Survey Misc. Field Studies Map MF—339. Rogers, W. P., Ladwig, L. R., Hornbaker, A. L., Schwochow, S. D., Hart, S. S., Shelton, D. C., Scroggs. D. L.. and Soule, J. M., 1974, Guidelines and criteria for identification and land use con- trols of geologic hazard and mineral resource areas: Colorado Geol. Survey Spec. Pub. 6, 146 p. 148 FLOOD. JULY 31—AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO _VBoulders in stream— ' 7. . ~bed derived from ' - ‘debris avalanche???” \ FIGURE 109.—Aerial view of debris avalanches from 1976 storm and flood, between mile 5.2 and mile 5.4 on the North Fork Big Thompson River. Debris avalanches were not common during the July 31, 1976 storm, but places susceptible to them are numerous in the Big Thompson area. The indicated features formed during the storm. Approximate scale: 1 in.=200 ft. Tweto, Ogden, 1976, Preliminary geologic map of Colorado: US. Soule, J. M., Rogers, W. P., and Shelton, D. C., 1976, Geologic Geol. Survey Misc. Field Studies Map MF—788. hazards, geomorphic features, and land-use implications in the area of the 1976 Big Thompson flood, Larimer County, Col- orado: Colorado Geol. Survey Environmental Geol. 10. Varnes, D. J ., 1958, Landslide types and processes in E. B. Eckel, Stokes, W. L., and Varnes, D. J ., 1955, Glossary of selected geo— ed., Landslides and engineering practice: Natl. Acad. logic terms with special reference to their use in engineering: Sci—Natl. Research Council Pub. 544, Highway Research Colorado Scientific Soc. Proc., v. 16, 165 p. Board Spec. Rept. 29, p. 20—47. A Acknowledgments .......................... Aggradation ................ Alluvial aprons ............................. Alluvial fans ............................... Dry Gulch .............................. See also Debris fans. Alluvial terraces, erosion .................... Alluvium .................................. Anderson, Larry W., assistance .............. Army Corps of Engineers, losses . . ...... Artificial fills ............................. Auto dealer, report, overbank deposition ...... Avalanches, debris ......................... B Balog, James, total sediment yield ........... Bar gravel, mile 48.3 ............... Bedrock .......................... . Big Thompson Canyon ...................... boulders ............................... flooding . . rainfall . .................. storm ......................... . . Big Thompson River ........................ aggradation ............................ alluvial fan .................... bank cutting . . below Drake, erosion .................... dam, Loveland ................. debris deposition .............. debris fans .................... desiccation cracks ...................... deflection .............................. discharge ..................... Drake, new course ............. entrenchment ................. . . flooding effects ......................... geologic hazard areas .................... gulch, mile 55.6 .................... .. Green Ridge Glade, overbank deposition . . Johnstown, tuff erosion ................. Loveland, lateral scour ........... Loveland Heights area, debris slide . . meanders ....................... . . mile 46.3, channel sour .................. mile 47.6, gravel deposition .............. mile 54.4, channel constricted . . . miles 15.7w0 ..................... miles 25-15.7 .................... miles 34.3—25.0 .................. mud cracks ...................... Olympus Dam area, erosion ....... . . 100-year flood plain ..................... overbank flooding ....................... pasture, erosion .................. riprap use ....................... Riverview Campground .......... scour ........................... sediment deposition .............. source .......................... .. streambed gradient . . ., ................. Sulzer Gulch ........................... 101 100 124 95 109 91 89 92 89, 95 100 138 INDEX [Italic page numbers indicate major references] Big Thompson River—Continued Page tributaries, erosion ...................... 92 truncation ............................. ' 94 Big Thompson River mouth, bridges ......... 127 channel modification .................... 126 Birch, river ................................ 94 Black Canyon Creek . . 89 Black Creek, scouring ........ . .......... 124 Bobcat Gulch, boulders ................. .. 139 debris fan .............................. 126, 139 Boulder Creek Granodiorite ................. 91 Boulders, Big Thompson Canyon ............. 92, 106 Big Thompson River, Olympus Dam area . 106 Bobcat Gulch ........................... 139 Drake ............................ 109 debris flows ...................... 102 flooding ...... . 139 Glen Comfort ........................... 103, 107 impact scars ............................ 107 Long Gulch .................... 109 mile 39.5 . . 117 mile 40.5 . . 115 mile 41.2 114 mile 43.2 113 mile 43.3 . 113 mile 44.3 . 110 mile 47.6 . 109 mile 47.7 ..................... 102 mile 56.2 ..................... 102 miles 55.6—50.9 ............... 106 movement defined ...................... 107 Noels Draw ............................ 105 North Fork Big Thompson River, gravel bars ............................ 124, 125 point bars .................. . . . 145 Bouldery point bars ......................... 145 Braided channels, Noels Draw ............... 105 Bridges, Big Thompson River mouth . . . . 127 destroyed, Bobcat Gulch ................ 126 mile 41.8 ............................... 113 mile 52.2 ................. . . . 107 miles 6.7-5.0 .................. 123 private, damage ................. 106 Buildings, construction comparison .......... 120 damaged. causes ........................ 138 Cedar Cove ................ 1 17 debris fans ................ 139 Devils Gulch ............. 120 fish hatchery ....................... 126 terraces ............................ 92 destroyed .............. 123 Drake . . 109 flood plain .......................... 131 mile 52.4 ........... 107 motel .............. l 10 Waltonia ........... 109 destruction causes ...................... 145 Building sites .............................. 138, 139 debris fans ........... 94 rockslides .............................. 102 flood damage ........................... 144 landslide destruction ................... 139 North Fork Big Thompson River ......... 118 protection .............................. 131 restaints, 100-year flood plain ............ 138 rotated on foundations .................. 131 C Page Canyon description, Big Thompson River ..... 91 Carter Lake reservoir ....................... 89 Casualties ................................. 88 Drake .................................. 109 Cattails .......... 127 Cattle ..................................... 136 Cedar Cove ............................... 91. 92, 115 buildings damaged 1 17 gravel bar .............................. l 1 5 Channel aggradation, Dry Gulch ............. 100 Channel blocking, trees ..................... 139 See also Log jams. Channel modification, Big Thompson River mouth .......................... 126 Channel scour .............................. 95 mile 46.3. Big Thompson River ........... 109 mile 52.2 ......... 107 Climatic differences ......................... 94 Cleanup, US. Army Corps of Engineers ...... 136 Colluvial materials, gullying ................. 139 Colluvial slopes ...................... 92. 144 Colluvium .......................... 92 Colluvium movement, mile 56.1 ......... 102 Colorado Geological Survey ................. 88 Colorado National Guard .................... 88 Colorado Piedmont ........................ 91, 94 Colorado State fish hatchery. damage ........ 126 Conclusions ............................... 140 Continental Divide ......................... 89 Corn ....................................... 94 destroyed, miles 34.3-25.0 . 134 Costs ...................................... 88 Cottonwood groves ......................... 94 overbank deposition ................... 131, 133 Covered Wagon Restaurant, mile 42.6 ........ 113 Cow Creek ................................. 1 19 Cretaceous. Upper. rocks .................... 91 Crocker Ranch, mass wasting ................ 95 Crop damage ................. 134 Crosier Mountain, sheetflooding ............. 124 Cultivation ................ Culverts, destroyed ......... Dunraven Glade ........................ D Dam failures, canyon mouth dam ............ 127 diversion dam, mile 43.25 ................ 110, 111 diversion dam, The Narrows ............. 117 Midway ................................ 109 Damage, flood. See Flood damage. Dark Gulch, debris deposited ................ 105, 107 scoured ................................ 103 streambed gradient ..................... 89 Deaths ................ 88 Drake .................................. 109 North Fork Big Thompson River . . 1 18 Debris avalanches ...................... 124, 138, 139 Debris bar, mile 41.3 ........................ 114 Debris deposition, Big Thompson River ....... 95, 109 Johnstown area ......................... 131 Debris dams ...................... 131 Debris fans ........................ 92, 95, 136‘, 145 Bobcat Gulch ........................... 126, 139 149 150 Debris fans—Continued Page building sites ........................... 94, 139 damage ................................ 138 destroyed, miles 45.4—46.3 ............... 109 flash flooding ............ 145 mile 3.7 .......... . . 125 Miller Fork mouth . . . . 124 Debris flows ........................... 101, 102, 124 Debris slides ............................... 101, 138 largest, Solitude Creek .................. 106 mile 4 .................................. 124 mile 56.1 ............................... 106 Noels Draw ............................ 105 Piper Meadows . 122 reactivated . . . 107 West Creek ................ 1 19 Deposition, imbricate structure .............. 106 Desiccation cracks, Big Thompson River ...... 130 Devils Gulch ............................. 89, 91, 119 fan .................................... 120 flooding ................................ 1 18 mile 1.2, sheet erosion ................... 119 Devils Gulch Road, destruction . 119 Discharge ................ . . 92 Diversion dam, mile 43.25 . . . 110, 111 The Narrows ........................... 1 17 Douglas-firs ................................ 94 splintered .............................. 103 Drainage ditches, erosion .................... 95 Drake ................................ 88, 89, 91, 109 Big Thompson River, new course ......... 109 bouldery alluvium ............. 92 deaths ........... 109 electric power out . 109 mile 44.9 ........... 1 10 North Fork Big Thompson River, dis- charge .......................... 1 18 property damage ....................... 109 preflood channel ........................ 92 Dry Gulch ............................... 89, 91, 100 sheet erosion . . . . . 95 Dunraven Glade ......................... 91, 123, 124 E Eagle Rock ................................ 100 Earthfill structures ......................... 138 Entrenchment, Big Thompson River . . 94 North Fork Big Thompson River . . 94 Erosion, drainage ditches ........... 95 north slopes ........ 139 sheet, Dry Gulch . . . . . . 95 Mount Olympus .................... 95 south slopes ............................ 138 total sediment yield ..................... 95 Estes Lake ................................. 89 Estes Park , . . 88, 119 altitude .. 89 moraines ..... 92 precipitation . . . 95 F Fall River .................................. 89 Fans ...................................... 101 Faults .................. 91 Thompson Canyon fault .. 91,92,1 10,114,1 15,125 Fences, flood heights ....................... 136 sediment traps ......................... 117, 123 Fish Creek ................................. 89, 91 Fish hatchery, damage ...................... 126 Flash flooding .............................. 109. 138 debris fans ............................. 138, 145 possibility .............................. 140 Flood damage . . ................. 136 bridges . .. . 106, 107, 113, 123, 126, 127 buildings ........................... 92, 107, 109, 110, 117, 120, 123, 126, 131, 138, 139, 145 grassy slopes ........................... 94 roads, dirt . 94, 95, 105, 119, 122, 123, 124, 139, 145 INDEX Flood damage—Continued Page trails .................................. 94 U.S. Highway I-25 ................. 128, 134, 135 U.S. Highway 34 ............ 89, 109, 117, 131, 133 U.S. Highway 287 .. ........... 127, 134 Flood deposits . . . 88 Flood frequency . 88 Flood plains ........ 89, 92 buildings destroyed . . . 131 Cedar Cove ............................. 115 Flood surface ............................... 144 Flooding, Big Thompson River, effects ....... 141 Front Range ............................ 140 height, fence posts ........ 136 Larimer County Fairground 134 minor, Miller Fork ........ . . 124 past ............... . 95 Foothills . . ....... 94 Forests ............................. 94, 95, 131, 133 See also Trees. See also Plants, forests. Forests slopes, sheet erosion ................. 95 Fox Creek, rainfall .......... 120 Frequency, flood ...... . . . 88 Front Range, flooding . . 87. 95, 140 storms ................................. 87 Fuller, H. Kit, assistance .................... 88 G Geologic factors ............................ 91 Geologic hazards ........................... 136 Geomorphic evidence .................. 95 Glaciers .............................. . 89, 92 Glen Comfort, boulders . ......... 103, 107 Glen Haven . . . .. . . 89, 91, 118, 119 rainfall ........................ 88, 120 Glen Haven picnic ground ................... 122 Gneiss ..................................... 91 Gradient, streambed, Big Thompson River . 89, 91, 100, 109, 114, 141 Grain sizes ................................. 88 Granite, rockfall .......... 103 Granite bedrock, scoured . . . . 103 Granitic grus ............................... 138, 139 gullying ................................ 139 Piper Meadows ......................... 122 Grasses .................................... 94 bent, Loveland area . . . 134 on fence posts ........ 136 on trees ........ . . 136 overbank areas .. 127, 128 sediment traps ....... . . 145 slopes, flood damage ................. 94, 95, 101, 105, 120, 122, 139 Gravel bars, Big Thompson River ............ 107 Cedar Cove ............................. 1 15 mile 41.0.... 115 mile 41.4 . . 114 mile 41.8 ........... 1 13 mile 44.1, Midway . . 110 mile 47.6 ........... . 109 mile 48.3 ............................... 109 miles 5.0-4.0 ........................... 125 West Creek ............................. 1 19 Gravel pits, Loveland ....................... 134 Great Plains ........................ . 89, 94 Green Ridge Glade, overbank deposition . . . 131, 133 92 Gullying ...... i. 92, 95, 101, 119, 122, 124, 139 Devils Gulch ........................... 119 Dunraven Glade, road ................... 124 Glen Haven ............................ 120 Miller Fork ............................. 124 H H bar G Ranch, sheetwash ................... 1 19 Hazards, geologic ........................... 136 Page Highway, bridge, mile 52.2 .................. 107 cuts, rockslides ......................... 102 destroyed. mile 44.3 ..................... 110 miles 42.6-41.5 .. 113 miles 44.0—40.0 . . 110 miles 47.7—44.9 . . 109 Hogbacks ...................... 91, 127, 129,131, 133 Hydrologic measurements ................... 88 I, J Imbricate structure ......................... 1 06 Impact scars .............. 107 International System of Units . . . 88 Irrigation, Big Thompson River . . . 91 Isohyet .................................... 141 Johnstown area, debris deposition ............ 131 turf erosion ............................. 1 28 L Lake Estes . 89 Landslides .................. 92, 95, 101, 110, 136, 139 Larimer County, structural damages ......... 88 Larimer County Fairground, flooding ......... 134 Lateral cutting, Dry Gulch .................. 100 Log jams .............................. 106, 114, 145 Long Gulch ...................... 89, 91, 105, 106, 109 Longs Peak ................................ 89 Loveland .......................... 126 Big Thompson River, lateral scour . 127 comfields destroyed .............. 134 dam .............. 91 grass, bent . . 134 gravel pits ............................. 134 mud cracks ............................. 1 34 pasture flooding ........................ 128 riprap .................................. 134 Taft Avenue ............... 1 27 Loveland Heights, mile 54.4 . . 103, 105, 106 storm center ................... 107 tributary gulches, scoured ............... 103, 106 Loveland powerplant, mile 41.3 .......... 1 13, 114, 127 M Markovic, William, assistance ............... 88 Masonry buildings .......................... 1 20 Mass wasting .............................. 95, 136 Meadows, H bar G Ranch, sheetwash ......... 119 Meander loop, cut, mile 5.3 .................. 123 Meanders, Big Thompson River . . . . . 127, 136 Mesozoic rocks . . . . . . . . . . ...... 91 Metamorphic rocks . . 91. 92, 117, 120 terrane, n rosion ...................... 92, 122, 139 Metric Convr ‘sion .......................... 88 Midway, mile 44.0 ........................ 91, 92, 110 dam destroyed .......................... 109 diversion dam, failure ................... 111 Migmatite ................................. 91 Mile 0.2, North Fork Big Thompson River . . . . 120 Mile 0.4, North Fork Big Thompson River . . . . 126 Mile 0.5, North Fork Big Thompson River . . . . 139 Mile 1.2, Devils Gulch. sheet erosion .......... 1 19 Mile 1.4, North Fork Big Thompson River . . . . 120 Mile 2.8, North Fork Big Thompson River . . . . 125 Mile 3.0, North Fork Big Thompson River . . . . 124, 125 Mile 3.7, North Fork Big Thompson River . . . . 125 Mile 4, North Fork Big Thompson River ...... 124 Mile 5, North Fork Big Thompson River ...... 124 Mile 5.1, North Fork Big Thompson River, de- bris avalanche .................. 124 Mile 5.2, North Fork Big Thompson River, de- bris avalanches ................. 124 Mile 5.3, North Fork Big Thompson River, me- ander loop cut ................... 123 Mile 5.8, North Fork Big Thompson River, tributary ....................... Mile 6.7, North Fork Big Thompson River . . . . Mile 7.3, North Fork Big Thompson River . . . . Mile 38.65, Big Thompson river, diversion dam ....................... Mile 39.5, Big Thompson River, boulders . . . sand deposition ......................... Mile 40.4, Cedar Cove ....................... Mile 40.5, Big Thompson River, boulders ..... Mile 41.0, Big Thompson River, channel scour . gravel bar .............................. Mile 41.3, Big Thompson River, Loveland pow- erplant ......................... stream gradient ...... Mile 41.4, Big Thompson River, gravel bar . . Mile 41.8, Big Thompson River, bridge ....... gravel bar .............................. Mile 426, Big Thompson River, Covered Wagon Restaurant .............. Mile 43.2, Big Thompson River, boulders ..... Mile 43.25, Big Thompson River, diversion dam . Mile 43.3, Big Thompson River, boulders ..... Mile 44.0, Big Thompson River, Midway ...... Mile 44.1, Big Thompson River, gravel bar . . . . Mile 44.3, Big Thompson River, boulders ..... highway destroyed ...................... Mile 44.9, Drake ............................ North Fork Big Thompson River ......... Mile 46.3, Big Thompson River, channel scour . Mile 46.9, Waltonia ......................... Mile 47.6, Big Thompson River, boulders gravel deposition ................. sandbars ............................... Mile 47.7, Big Thompson River, boulder move- ment ........................... Mile 48.3. Big Thompson River, bouldery grav- el har ........................... stream gradient ........................ Mile 49.9, Big Thompson River, surficial de- posits .......................... Mile 50, Big Thompson River, rainfall. Mile 50. 9, Big Thompson River, gulch scouring. Mile 52. 2 Big Thompson River, channel scour . surficial deposits ........................ Mile 52.4, Big Thompson River, house de- stroyed ......................... Mile 54,4, Loveland Heights ................. unnamed gulch, debris deposited ......... Mile 55. 6, Big Thompson River, gulch. gulch scouring .................... Mile 56.1, Big Thompson River, colluvium movement ...................... debris slide ............................. Mile 56.2, Big Thompson River, boulder move- ment ........................... Miles 3.0- 4. 0, North Fork Big Thompson River .......................... Miles 5. 0- 4. 0, North Fork Big Thompson Riv- er gravel bars ................... greatest damage ........................ Miles 5.2—5.4, North Fork Big Thompson Riv~ er, debris avalanches ............ Miles 6.7-5.0, North Fork Big Thompson River ........................... Miles 15.7-0, Big Thompson River, overbank deposition, zone 4 ............... Miles 25-15.7, Big Thompson River .......... Miles 34.3—25.0, Big Thompson River, corn destroyed ....................... overbank deposition, zone 2 .............. slump bedding .......................... Miles 37-343, Big Thompson River, overbank deposition ...................... Miles 38.9-37, Big Thompson River, The Narrows ........................ Miles 38.9-40.4, Big Thompson River, Cedar Cove ........................... Page 124 120 122 117 117 117 115 115 115 115 114 114 114 113 113 113 113 111 113 110 110 110 110 110 109 109 109 109 109 109 102 109 109 102 100 106 107 102 107 103 107 107 106 102 106 102 125 125 125 139 123 136 135 134 133 134 129 117 115 INDEX Page Miles 41.0—40.4, Big Thompson River, channel narrow ......................... 1 1 5 Miles 42.6-41.5, Big Thompson River, highway destroyed ....................... 1 13 Miles 44.0—40.0, Big Thompson River, highway destroyed ....................... 1 10 motel destroyed ........................ 110 powerplant destroyed . 1 1 1 restaurant destroyed .................... 110 Miles 45.4—46.3, Big Thompson River, debris fans, destroyed .................. 109 Miles 47 .7—44.9, Big Thompson River, highway destroyed ....................... 109 stream gradient ........................ 109 Miles 52.2—49.9, Big Thompson River, surfi- cial deposits .................... 102 Miles 56.2-47.7, Big Thompson River, mass movements . . ................ 101 Miller Fork ............ 124 Montane forests. 94 Moraines, Estes Park. . . . . 92 Motel destroyed, miles 44. 0— 40.0 ............. 110 Waltonia .......................... .. . 109 Mount Olympus ............................ 95 Mud cracks, Big Thompson River ............ 130 Loveland ............................... 134 Mummy Range ............................. 89 N Narrow-leaf cottonwood ..................... 94 Narrows, The ........................ 91, 92, 117, 129 diversion dam .......................... 1 1 7 Noels Draw ................................ 105 lateral cutting ................. 107 streambed gradient . . .......... 89 tunnel tailings ........... 105 North Fork Big Thompson River . 89, 118 aggradation ............... . . 94 deaths ................................. 1 18 debr1s avalanches ....................... 124 debris fans ............................. 92, 124 deflect1on .............................. 94 Drake, discharge . . ............... 118 entrenchment . . ............... 94 geologic hazards . . 138 greatest damage . . 125 mile 0.2 .......... . . 120 mile 0.4 ................................ 126 mile 0.5 ................................ 139 mile 1.2, deposition ..................... 119 mile 1.4 ................................ 120 mile 3.0 . ............... 124, 125 mile 3.7 . ............... 125 mile 4 . . . 124 mile 5 .............. 124 mile 6.7 ............ 120 mile 7.3 ................................ 122 miles 3.0—4.0 ........................... 125 miles 5.2 5 4 debris avalanches .......... 139 miles 6.7 5 0 ........................... 123 road, new river channel 124 storm ................ 88 streambed gradient . 89 truncation ........................ 94 North Fork Big Thompson River mouth, mile 44.9 ............................ 109 North slopes, erosion effects ................. 139 0 Olympus Dam ........................... 89, 91, 100 Olympus Heights ........... 89, 95 Olympus tunnel ............................ 89 Omaha District, U.S. Army Corps of Engin- eers ............................ 88 100-year flood plain ......................... 136, 138 legal definition .. ....... 136, 138 Orographic controls ......................... 95 151 Page Overbank deposition, Big Thompson River . . . . 129, 145 Green Ridge Glade ...................... 131, 133 Larimer County Fairground ............. 134 miles 37—343 ........................... 129 mud cracks ........ 130 U.S. Highway I—25 .. 135 U.S. Highway 287 134 zone 1 ........................ 129 zone 2 ........................ 133 zone 3 .................................. 135 zone 4 .................................. 136 Overbank flooding, Big Thompson River ...... 128, 129 Overhead siphon, The Narrows .............. 117 P Paleozoic rocks ............................. 91 Park, defined ............................... 89 Parnell, Roderic A., assistance ............... 88 Past flooding ......................... 95 Pasture erosion, Big Thompson River . . 127 Pasture flooding, Loveland area ........ 128 Pegmatite dikes ............... 91 Photographs, postflood . . . . . . 127 preflood ................................ 127 Pine trees, bark stripped .................... 131 sheetflooding ........................... 94, 145 Piper Meadows ........................... 92, 94, 122 Plains area, pasture flooding . . 128 Plains grasslands . . . 94 Plants ........... 94 cattails ........ . . 127 com ............. . . . 94, 134 cottonwood groves .................. 94,131,133 Douglas- fir ............................. 94, 103 forests ......... 94,95, 103, 117, 123, 131, 133, 145 grass ................. 127, 128, 134, 136, 139, 145 log jams ........................... 106, 114, 145 narrow-leaf cottonwood . ........... 94 pine trees ............ 94, 131, 145 Ponderosa pine ....... 94 Rocky Mountain juniper , . . 94 sugar beets ............................. 94 willow ................................. 94 Pleistocene, glaciers ........................ 92 Point bars . . . .......................... 100 Ponderosa pine ......... 94 splintered .............................. 103 Powerplant destroyed. miles 44.0—40.0 111 Precambrian rocks ......................... 91 Precipitation. See Rainfall. Preflood photographs ....................... 127 Property damages .......................... 88, 109 See also Buildings. Q, R Quaternary deposits ........................ 91 Rabbit Gulch, erosion ............... . . . 105 Rainfall, Big Thompson Canyon . . 100 Estes Park ............................. 95 Fox Creek .............................. 120 geologic hazards ........................ 138 Glen Haven ............................ 120 intensities.. 88, 92 mile 50. 100 Olympus l-Ieightsu 95 Piper Meadows ................... 122 References ........................... . 146 Regolith ................................... 92 Residential developments ................... 139 Restaurant destroyed, miles 44.0-40.0 ........ 110 R111 erosion .................. 92 Ripples .......... 134 Riprap, Loveland ........ 134 Riverview Campground, erosion ............. 127 152 Page Roads, base, US. Highway 34 ............... 89 Devils Gulch Road . . . 119 dirt, flood damage . . . 94 gullied ........................ 95, 122 Loveland Heights, gullied ............ 105 scouring ............................ 145 West Creek, erosion ................. 119 embankments, erosion .................. 92 landslide burial ......................... 139 miles 6.7-5.0 ........................... 123 new river channel, North Fork Big Thomp- son River ....................... 124 Piper Meadows ............ 122 sheetflooding damage ....... 139 US. Highway 1-25 ................. 128, 134, 135 US. Highway 34 ............ 89, 109, 117, 131, 133 US, Highway 287 ...................... 127, 134 Rockfalls .............................. 101, 138,139 Glen Comfort ........................... 103 miles 41.0—40.4, Big Thompson River ..... 115 Rocks, Boulder Creek Granodiorite ........... 91 gneiss ................................. 91 Granitic grus ................... 122, 138, 189 metamorphic rocks ...... 91, 92, 117, 120, 122, 139 migmatite .............................. 91 pegmatite dikes ......... 91 Precambrian ........... 91 schist ................ . .. 91 Silver Plume Granite . . ....... 91, 92, 103 Rockslides ............... . . . . 101, 102, 138 Rocky Mountain juniper .................... 94 Rocky Mountain National Park .............. 89 S Sand deposition, Big Thompson River, mile 39.5 ............................ 1 17 zone 1 . zone 2 . zone 3 . Sand sheets ................................ Sandbars, Big Thompson River, Olympus Dam area ............................ 106 Dry Gulch ...... . . 100 mile 47.6 ........................ . . 109 North Fork Big Thompson River, miles 6.7-5.0 ......................... 123 Schist ..................................... 91 Scouring, locations ................... 92, 95, 125, 144 Sediment deposition, Big Thompson River . . . . 109 grain sizes .............................. 88 local derivation. zone 3 .................. 136 Sediment traps ........ 94, 117, 120, 123, 131, 133, 145 Sediment yield . . . ..................... 92 Shear zones . . . . ............. 91 Sheet erosion .................... 94, 95, 100, 119, 136 Sheetflooding .......... . ................... 92 Crosier Mountain ................. 124 Dunraven Glade .................. 124 pine forests .......... . 94, 145 Piper Meadows ...... 122 scour ........... . 145 valley slopes. .............. 139 Sheetwash ........................ 95 H bar G Ranch ......................... 119 INDEX Page Sheriff Bob Watson ......................... 88 Silt deposition, zone 1 ................... . . 129, 130 zone 2 .............................. . . ‘ 133 zone 3 .............. 135 zone 4 ............... . . 136 Silver Plume Granite . . 91, 103 terrane, erosion ...... 92 See also Granitic grus. Siphon, overhead, The Narrows .............. 117 Slope failures ............................... 101, 139 See also Debris flows. See also Debris slides. See also Rockfalls. Slope wash ................................. 92, 105 Slump structures . . . 131, 134, 136, 138 Soil, A horizon . . ....................... 105 classification . . ....................... 88 description ............................. 92 north slopes ............................ 139 profiles ......................... 92 south slopes ....................... . . 138 Solitude Creek ................... 105, 106 South PlatteRiver... 89,91, 126,128,141 flood rise ........................... 1 36 South slopes. erosion effects ................. 138 Southern Rocky Mountains .................. 94 Storm, area ................................ 89 center .................................. 107 damage. See Flood damage. debris fan effects ....................... 138 development, 2100 MDT . . 109 rapid growth ............. 140 recurrence . . . ....... 88 summary .............. 89 Streambed gradients ........................ 89 bedrock influences ...................... 91 Big Thompson River .................... 89 Dry Gulch .............................. 100 flooding effects ..... 141 mile 41.3 ........... 114 mile 48.3 ........... 109 Streambeds, overbanks ..................... 94 Structural damages ......................... 88 See also Buildings. Sugar beets ....................... . . . 94 Sulzer Gulch, Big Thompson River . . . . 130 Surficial characteristics. granitic grus ........ 139 metamorphic rocks, weathered ........... 139 Surficial deposits ........................... 91, 92 miles 52.2—49.9 ......................... 102 T Taft Avenue, Loveland ...................... 127 Terraces ................................... 91 erosion, building damages ............... 92 Thompson Canyon fault ...... 91, 92, 110, 114, 115, 125 Thunderstorms, debris fan effects ............ 138 rapid growth ........................... 140 See also Storm. Timberline ................................. 89 Trails, flood damage 94 Page Trees ........................... 94, 95, 103, 131, 145 channel blocking 139 cottonwood ......................... 94, 131, 133 debris flows, scarring .................... 102 Douglas-fir ............................. 94, 103 logjams ........................... 106, 114, 145 pine ............ .. 94, 131, 145 Ponderosa pine ......... . . 94 Rocky Mountain juniper ................. 94 sediment traps ..... 94, 117, 120, 123, 131, 133, 145 willow ......... 94 zone 4 .................................. 136 Tributaries, erosion ......................... 92 Truncation, Big Thompson River . . . 94 North Fork Big Thompson River . . 94 Tunnel tailings, Noels Draw ........ 105 Turf erosion, Johnstown . . . . 128 South Platte River ...................... 128 U United Soil Classification System ............ 88 US. Air Force .............................. 88 US. Army Corps of Engineers, cleanup ....... 136 losses .................................. 88 US. Bureau of Reclamation, soil classification 88 US. Highway I—25 ......................... 128 farming area .. .. . . .. 134 overbank deposition . .. 135 US. Highway 34 ........................... 131 Drake .................................. 109 road base .............................. 89 sand deposition ......................... 133 The Narrows .......... 1 17 US. Highway 287 ......... . 127 overbank deposition .................... 134 V, W Valley slopes, damage ......... . . . 139 sheeti'looding ........................... 139 Vegetation. See Plants. Waltonia, mile 46.9 ......................... 89, 109 motel destroyed ........................ 109 Waste disposal systems ..................... 91 Water availability .......................... 138 Watson, Sheriff Bob, aid 88 Wells ........... 91 West Creek 119 building ...................... 120 tributaries, flooding . .............. 1 18 Willow ..................................... 94 Z Zone 1, flood plain, debris deposition ......... 131 Zone 2, overbank deposition ........ 133 Zone 3, overbank deposition ...... 135 sediment, local derivation . 136 Zone 4, overbank deposition . . . 136 ads, GOVERNMENT PRINTlNG OFFICE: 1979—677—026/4 UNITED STATES DEPARTMENT OF THE INTERIOR PROFESSIONAL PAPER 1115 GEOLOGICAL SURVEY PLATE 1 1 7 10530 2230" > 15' 7'30" WW 10999} 5230" W Loveland Greeley $3 ' a 40° 30! 777,7 77, 77 77777_‘7 V , 40°30, 7 ESTES PARK 9 GLEN HAVEN DRAKE MASONVILLE LOVELAND WINDSOR DENVER ‘ QUADRANGLE 4’ : QUADRANGLE OUADRANGLE Ge” OUADRANGLE OUADRANGLE OUADRANGLE \__ 7/9,‘ ‘ 2’3} D O “W: S’rcym 6 ¢ (3 ’/,‘\ M un’rain 2/ 04 A I} ‘ / 2,3 000 o? g: 9 // Pueblo A ”0,6 In % / on 2» 4 0/0 9 as a E ' / \ J\\ Fox v- ' \. . C J 7 Peek /Ie \ _ N0, ‘ k 45, 104°37'30" 5 even 20 ‘8» "i BRACEWELL ‘ GREELEVw \/\ Wes, Creek 57/ ‘ pa 0 QUADRANGLE OUADRANGLE / 'I P o (:3 _— alisade -\ o I'— creek S’ IE Piper 31m Mountain ‘3, ‘3: ® I cow 06‘ Meadows MI_ firesiap " J , ’ 3/ \ oun am I, __ / ' \ ”he Alexande ’u J 960“ ' r— H bar G <0 MIn 06‘ Ranch 0 \G‘”: ‘3: I ‘ \Q§°\ GREEL: I & f ‘0 g (3 I 09¢ 00/06 In eé -;_ g |_ Q\ 5 9\6 E Riverview j I Eagle 3 f , E : Campground I Q \‘5 Rock 0 F S 2 Q73" 0° I / E ,9 E | | 4 x O : L / (3/ E J'-Mariana \ , T 4OMPSO/V ‘ I \s 2,0 ( 2r ,. \ | ~1\ 3° 9 "° ' ~ , Bufie u e __._\ “‘/ 2 e3“ \ 50°“ f I (ea “ ' \ («w // HeighTS $5 9 04 E ‘ O \eodfiq‘ B I G M0un1 6‘ : DIV I ; L 5;, \u 1,,Olympuséo 0/0200 \ : .\/\ a u .’ -.,,,,, : . ,, ,- , ,, «W ., , 2230” 40 22130 . V I“ 7 fiwi 77‘7777 . 74— ”” ,,,,,, "*Errrwrwwwr'fl'fi " " "'"' " ' "" " " ’ W "" ’ ' "W" 7 V l I > LA SALLE . PANORAMA PEAK P/NEWOOD LAKE I CARTER LAKE BERTHOUD JOHNSTOWN I «9 ~ I a; UADRANGLE I QUADRANGLE RESERVOIR OUADRANGLE OUADRANGLE ‘ GUADRANGLE; ‘ ~ \ §\. —,‘," ".Igoum °a= Norm F0”, QUADRANGLE 1 ‘ ~ k ’u Is h ~ 9“) , 2.352153%} 9 9° "ff/e Thompson my” I . ‘ 4, : Panorama LONGS PEAK I \1' EN“ PW“ LARIMER COUNTY OUADRANGLE \ WELD COUNTY : \v 105°30’ 22,30” 15’ 7'30” I 4 O o l 5 I 377L¥,¥W~7 77 _,L,,, ,,i ,_,,I 4015' 1’OS°OO' 52'30" 104°37'30" MAP OF BIG THOMPSON RIVER AREA SHOWING LOCALITIES AND GEOGRAPHIC FEATURES MENTIONED IN TEXT, 7V2-MINUTE QUADRANGLES USED IN MAPPING 1976 FLOOD EFFECTS, AND FLOOD STAGE AND DISCHARGE DATA SITES (BOXED NUMBERS) QUS, GOVERNMENT PRINTING OFFICE: 1979—677-026/4 U 1 2 3 4 5 1'0 MILES l_ I | I I I UNITED STATES DEPARTMENT OF THE INTERIOR ' PROFESSIONAL PAPER 1115 GEOLOGICAL SURVEY PLATE 2 (SHEET 1 OF 2) 2230" 7 105°32'30" 27,30” 105°20’ \i/l: < e f \‘V a . s ‘7 F , ,/ , » ‘ ./ K; , IR \ x7: ._ __ , f ., .. s; \ :\ / 40°3o' W77 77 . _, _ _ 40°30' T / ' x «(I __ , / _ ~. T “* _ T" A \nggwo / / ~ - . , i . _. .\°\\ . \ EXPLANATION // / 'A L“ / , . _ s. . , i _ 7 , _ /’7.7: \ ' _ . \ A. \> _ > 1;, _ X \ , \ __ \ g/ \\;,:/‘ ' g I9 ‘ - ‘ ' -_ -‘ -- ‘ . “ X , -. . ~ 1 l \ , tL » \‘ . . {UN \ _ ® , I 3% EROSION AND DEPOSITION ON SLOPES MODERATE EROSION AND DEPOSITION—Characterized by continuous a . c . ‘ .y , , 2 . \\ \ ‘ g \L \ . .. . . . I . \'\‘i. _ ~ 1 /: GULLIES—Cut in thick sandy slope wash deposits on grassy slopes and in “M“W“ channel scour and deposition of sandy to bouldery material eroded from / / \ thin stony colluvium over bedrock on forested slopes. Gullying most the bottom and srdes of the gulch. Bedrock floor of gulches locally exposed. :; extensive along dirt roads, roadside drainage ditches, and in preexisting Includes short segments With minor ”051°“ and deposition 01’ major (”0510“ / gullies. As large as about 600 ft long, 7 ft deep, and 20 ft wide on gentle and local deposition. Overbank sediments are mostly sands to pebble grassy slopes and commonly cut to bedrock (generally less than 6 ft) on gravels. Colored area represents approximate extent 0f flOOdan; longs ’t’ steep forested slopes. Small gullies not shown in densely forested areas , . “ dashed red line used where channel is too narrow to show actual Width Z A l LANDSLIDES—Mostly developed in thin colluvial mantles over bedrock, j,» MAJOR EROSION AND LOCAL DEPOSITION—Characterized by , primarily on north- to northwest-facing slopes of about 60 to 85 percent. I continuous deep channel scour and by local deposition 0f bouldery gravel 3; Includes: rockslides (RS), rocktalls (RF), debris slides (DS), debris flows mostly at insides of sharp bends. Much of the bedload and bank materials fr (DF), debns avalanches (DA), and undifferentiated landslides (LS). Tail on less than about 3 ft in diameter commonly washed out of the gulches. Bedrock floor of gulches is exposed; locally manfled by thin discontinuous (I, flood deposits and lag boulders. Overbank sediments are mostly sands to pebble gravels that locally contain some cobbles and boulders. Colored 3" area represents approximate extent of flooding; solid red line used where - triangle indicates mapped upslope extension of deposit or associated erosional scar SHEETWASH DEPOSITS—Composed mostly of very fine to very coarse pebbly sand, commonly less than 4 inches thick. Locally may include _ . . . channel is too narrow to :show actual width \ x thicker, coarser grained deposrts near the mouths of gullies and small gulches. Usually eroded from steep forested slopes and deposited on EROSION AND DEPOSITION ALONG MAIN STREAMS x,‘ adjoining gentler grassy $101385 Areas Of sheet erosion, WhiCh are mostly Minor to moderate channel scour within floodways of Big Thompson River , limited to forested slopes, are not shown and North Fork; intensive scour restricted to outsides of relatively sharp V - Deposits of sand to pebbly sand that accumulated in areas near the mouths of bends and areas along constricted reaches. Channel deposits range from L gullies and small gulches, where the mode of flow changed from confined to sand to boulder gravel; overbank sand to pebbly sand occurs locally. ”T unconfined and in areas where the flow was impeded by obstructions such Colored areas represent approximate extent 0f flooding f as fallen trees, road embankmentsi and fences. Locally may include some Widespread deposition of sand to pebbly sand. Includes some small deposits i coarser grained deposits with a small amount of cobble-size material and ' of pebbly to cobbly gravel. Dotted red line used along North Fork above QC some sheetwash deposits. Usually less than 5 ft thick Glen Haven where channel is too narrow to show actual width : in III III III Areas of varied erosional and depositional features too small, discon— Fine stipple indicates areas of widespread deposition of pebbly to cobbly _—_-_——_ "T:- tinuous, or indistinct to map individually. Includes one or more of the gravel; includes some small deposits of pebbly sand and cobbly to bouldery f following: shallow SUIIIBS, sheetwash dePOSltSi areas 0f bent grass, or gravel. Coarse stipple indicates widespread deposition of cobbly to gravelly lag deposits bouldery gravel; includes some small deposits of pebbly to cobbly gravel 14 EROSION AND DEPOSITION IN TRIBUTARY GULCHES —— Local areas of intense lateral scour MINOR EROSION AND DEPOSITION—Characterized by discontinuous to ! Extensive channel scour withn floodways of Big Thompson River and North \ continuous shallow channel scour and deposition of sandyto cobbly mate- Fork' deposition of COb)I€ to boulder gravel. Bouldery gravel bars rial eroded from the bottom and sides of the gulch. Bedrock floor of gulch common especially alonr insides of bends. Overbank sands to boulder ’T not exposed. May include short segments with moderate erosion and ’ ‘ teposition. Overbank sediments are mostly sands to pebble gravels. Col- ‘ bred area represents approximate extent of flooding; short-dashed red line used where channel is too narrow to show actual width gravels occur locally. Cdored areas represent approximate extent of : flooding. Fine stipple indicates areas of widespread deposition of cobbly to f bouldery gravel; includes iome small deposits of pebbly to cobbly gravel. Coarse stipple indicates widespread deposition of bouldery gravel; includes some small deposits of cobbly to bouldery gravel my») 7 7' h \afi Valli 2 ._ Q I ‘ __ massx\ , 631% \1 a, l. Intensive channel scour throughout floodways of Big Thompson River and North Fork; local deposition of cobbly to bouldery gravel. Much of the 72 sandy to bouldery material scoured from the channel was deposited farther »-: downstream along reaches with lower gradients. Overbank sands to \ boulder gravels occur locally. Colored areas represent approximate extent i of flooding. Coarse stipple indicates local deposition of bouldery gravel 127'30" ’ w Segments of highway severely eroded or destroyed by floodwater 1 ‘ 0—3533 Boulder—measurement site—Number after letter B corresponds to riVer Iii mileage of site. Sizes of largest boulders are summarized in tables 7 and 8 in N text 0-— ND/ND/ High water and scour measurements on intermittent and perennial I 30/ 8 br streams—Numbers, left to right, represent width of floodwater, height of floodwater above present floor of gulch or present stream level, postflood / width of channel, and postflood depth of channel. All measurements are in ’/ feet. Letters ND in place of measurement indicate not determined. Letters 1/ T B N br after depth of channel indicate that the channel was scoured to bedrock * T. 5 NI. 0 3, 2 IO ft 3/5 Peak-discharge measurements (in cubic feet per second) by Water Resources Division of the U.S. Geological Survey / ‘ MG W Isohyets (in inches) for July 31—August 2, 1976 storm, slightly modified from I unpublished data provided by the National Weather Service , “a l'QI Mileage marker—Indicates distance in miles along riverbed , ’i t... _ mil 4/ .\._ TEN. TEN, ms TN .. Lee y) y w ’ CONTINUED ON PLATE 2—B (Sheet 2) 25' C. , _,e/ i C ’ A I” ., (meat—3. \ \3 ‘ {iv " \_ V". . IQ’Q , Role 868.? -_\< LII—III II 22*30" g ' g, . . ,. - , " . “J "a ' I ,t . t, ' s . " l ’7 1 , e ‘7 ‘x . ~ _ i ‘ \ \ L ’ W l t _ " a ‘ , , , . / ; t." .H, -_-,,-‘,e , Y< . . ' ’ 1 r ,‘ f - \ .. \ ,, , ‘ ._ I; . ‘ y ,K D i. ,7 a; X , _ _ , C .. H‘ . i 7‘ . it . *. L .. _> s , . 22.30" l T " t “x?! ”a e l l I‘l / ,l ._T‘: I \ "3' fa .\ I Vfi\ \ . T 5N. BMW/f; T.4 N. ' T.5N, T.4N, I :— ‘* L ,, ' VII" ~ \ Li R\\ ‘\_ P a Y R\ I 40020! ‘ ‘, I I‘ """" 40°20' , NOTE Much of the mapping on plate 2A—C is based on the interpretation of postflood stereographic aerial photographs, including black and white imagery of the area along the Big Thompson and North Fork Big Thompson Rivers (approxi- ; _~ . mate scales 1:3,500—1:8,000 by Kucera alnd AssOciates, Denver, CO, and 1 l) ._ f \ {I i 1:6,000 by Hogan/Olhausen, Loveland, CO) and infrared color transparencies j 'i\“\ sit/Lye) ‘j,\\_\ " , of the mountainous part of the study area (approximate scale 1:24.000 by. ‘ ‘\\fx.j,2 It I, T. lntrasearch,Denver, CO). Aerial-photograph interpretation was supplemented ' , by field observations mostly along the main streams, in some ofthe major and E minor tributary gulches, and in the Estes Park area. / " .. ‘ - . W\\ [N3248 » ~ _ \lll (ENDED Om>> w 105°32'30" 1“ ‘ ‘5‘ i \ i ’ «5 / n /// R. 72 w. H, 71 w, 105° 20' Base from U.S. Geological Survey, EiafiffiijEtGZEE'nkégf’géfiZd[132??an A. BIG THOMPSON RIVER FROM ESTES PARK TO DRAKE AND NORTH FORK BIG THOMPSON RIVER FROM GLEN HAVEN To DRAKE coLo:ADO :Il’fléffegttsnri‘gfiglsby W" 8cm“ GEOLOGIC AND GEOMORPHIC EFFECTS OF THE 1976 STORM AND FLOOD, BIG THOMPSON RIVER AREA, COLORADO APPROXIMATE MEAN QUADRANGLE LOCATION DECLINATION, 1979 SCALE 1:24 000 1 . . , , 1/2 | . 0 lMlLE I I i i i 4 1 .5 O 1 KILOMETER i——l l—l F—l l—l l—l l-——-——-——————-—l CONTOUR INTERVAL 40 FEET NATIONAL GEODETIC VERTICAL DATUM OF 1929 nuts. GOVERNMENT PRINTING OFFICE: l979—677.026/4 UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY PROFESSIONAL PAPER 1115 PLATE 2 (SHEET 2 OF 2) 105°20' R.71W 17'30” H.68W. 105°02’30” 40°27'30" . m I ,I I , _ y m I 4ou27'3o" CO ‘ O . O \7 TEN. SESHOE' _ ‘97.: ,‘ - r‘ 5., /il :4 \f' I. I.:',. )5 / jrvdLI / d . ' 7‘ 5' 1.; 31,20 ft,I3/’s_ : . ‘l ‘17”3Tj‘1v/5\\ ‘1“ “L 13““ L L E (77 I 4 , m A \ a 5’ FT» \ V L) w > T A! E ”Ii , D I L; I 5 l _l I \ n. r n. ,1 y M , 2 g 7‘ il\ L114\ <“L\ . O T Overbank-sedimentation zone 1: Fine to very fine sand; ' * < _ I . - , 77 ‘ II D B ' cobbles, pebbles, coarse to medium sand locally near - '/L ‘I :I g . ' ' . ' I , . i "L- ’ 3 bank edge, very little srlt, much mlca. MaXImum / ,I _ \ . . , UN” E Z thickness about 6 ft; large lateral and downstream / , 7 ‘ a . , I, ‘ ,, A ‘_ . ~ . : L . i _ 7 F ’ '2 variations in thickness. Structure is high—velocity flat \ ’ . - .. \ t " _ , R J ‘ 7/ '. N) ‘ I I ‘ _ L A K E (23 O bedding; very local foreset and slump structures . _ ’ “v- 7 , ‘ « . I, , . .I . . I y 7' . » o ;.LOVELAND:‘ W , . i V I Y TL L t ,7; a ' > LL L L L L‘ T L i L L - L L L ‘ LL 7 / _. \ L \ L I. - L . 'L * - ‘_.\ I‘L LL ' L ' ‘ ,L L‘LII‘W ._. ‘ 44 II .I : '1' I '/ . I’ LLL'\ I L" L ”I I . L ‘I _ T 7 7 'L I‘ .- ' .. n. .; , n I I IE; L .. IIng Flame- 3 A l A . \ I. L \ I 5004V . --”X6996 v' L L _ ’ , L L L \- Li‘ LIL I . L 4 L L. LL - L‘ “L3?" " , , ' . ‘ ,, , . I , I «’- 1: I7 , , I . _ . , 7 ‘Il/I Overbank-sedimentation zone2:Sandgradingtosiltaway . ~ ._ 7." I , ‘ ' ,, \ . , .L‘ 4 - I I I 7 :2 . , , , , ,I; 7’ \ :_\ , \_:\ \Q \\ .1, ' \L/ -\ ‘* I/r’x \ - \ \ I \ , 2 7/17 > J ‘. I, ‘ . an; s I, ‘ \ /‘ I I \ 4. :4 - [I __ , 1.. 1:“ .3 \ from river; abundant mica. Nearly everywhere less ~ v L L L'LL L‘ ‘ ' ‘ L ’L L ~- L LL . F ‘ \ \' ‘ ‘ ‘ L ' 9 TL " ‘ "I L 'L I ’ L L ~ 7, ‘ * .3 . I " 7 .4 ‘ , ‘I than 2ft thick; thickness variations less than in zone 'L ' 1. Structure is flat bedding; minor slump bedding and Iv __. H I _ BOEDECKER . v \ .7 _-__:_. ./ _ 4 .\ ., . . ‘ 7I. _ , _ 4 __ 7/ ‘ ‘ _ I? \477 , .7 .7 I, , , 4/ I, _‘ : , .0. 7- :7; v.1! 7 . j 7 4. \ 7' [\7, K J , (y - I j I ‘ 7 . . K . IV \ r ‘0 \ _\ . ¥ ‘7 ‘_ fl "pm 4. 7 , / I, "l , > I. L A K E G ‘15 ’ :1 ’ _\ -..\ V \4 i . I_\ . ,‘ 7 > 4 _ . : / \ . x}: /‘ / 7/ :7: =4. (7, , / 7 4 T I , :7: ‘ ,3 _ H ‘ /. . \x ». ‘ , \ . 7 , 7 ‘ y “4 7 I, . -_ I7 -_ ,I - - ‘ ' . __ _77. » 4g __ / ' . \ _I . / I ’7’ -. ‘ ' /, g, y . 3 L ’,_ ,4 7I ” g I I J ,4\ 7 , f 7‘ 4“ “’ I \:/ 7/4 L ' -._ , ‘ .\__ L I» . , . L I.» ‘ ‘I ‘ -‘ l \ O s 4 ' . I' ~ ‘ ‘ ‘ ‘ ‘ / ' 7 I -_ / I ‘ \ ‘ ‘ ‘ I I I ./ ~ 77a , I , -. K . ‘II , \ _ I :1 7 ‘ \ 7 \ I\ Y 7 _ \ 77 . ‘ : . , :_ . : ‘ 7. 7 ‘ l . Kings ~ ' . H , , :III . .. . _ \ . r 1 ‘ L’ ‘ 5' ’ L Corner 255 _ I 7 , __ _7 I . , , 7 7 \ 1,; l I. \11 I / II I 3. , . , my,“ 2 _ .7 7 .. -- ,, - 7 -, . 7. :5 1 - - .. 9- I 'Holowell '- r f _ ~ , __ ‘ 7 3: _ I, __ __ 7 [I 1‘ , : I/ , ‘ .. ‘ 4. ._ .fi, ‘ I g L' \ “ L‘:\“I\\\L'§ L \‘ILILI L L L L L L LL ‘ COLL?” r 40°22'30” L , L L ' 7'7! . F 4:+_, , . I! . . ,7 , , x , , I _7 ,, - LiL . 5‘7 7 I ’ - ”j, _ I :/ - 7 .. 7< I .' "0 L\ , n . ,4, 7 105° ' i LOVELAND /8 MI _ _ f " ’ ’ ' ' ' ' L ' " T "T ‘ ' f - x l - \ , a 4 \ A ' .. .. .1 . , g, . . 7 L7 7 7 '~ 7 77 < , l [(3 7 77 .. . 4 .. , ., ,. . 7 77 y 7 : . 7 . L , ‘ . , .11” ‘ 1 v_ ‘ . . ‘ A . . CAMP/ON 4 6 M} T 0730” /CAMPION 3 MI, 5’ bAMP/ON IOOLO. 50; 2.2 MI, Hsgw H 68W. 10500310353 30 ' BERT ‘ 3. BIG THOMPSON RIVER, EAST OF DRAKE TO LOVELAND HOUD 5 MI. A A rO o I» r , EXPLANATION 105L02L3OLL 9- 58W o A Landslides—Includes rockslides (RS) and debris slides (DS) 40325, . . \ 105 57130” 104°52’30” 40°25, EROSION AND DEPOSITION ALONG MAIN STREAMS —% I.‘L /._ l ‘ I7_\ (I \\ W /,.//LL V ‘I \”44‘ Minor to moderate channel scour within floodway of Big Thompson River; intensive scour restricted to outsides of relatively sharp bends and areas . ‘IU/ /7\\_1 7.//’LLLLLL//T / _ ”xv. LL/FT. /1’ along constricted reaches. Channel deposits range from sand to boulder / ~-\.\\‘//#— \7 y [I - . _- \Y‘ ‘ \ '\ ‘I.’ .l ”i \\_ gravel; overbank sand to pebbly sand occurs locally. Colored areas repre- k if) __ 2 5L sent approximate extent of flooding g 2' - Widespread deposition of sand to pebbly sand. Includes some small deposits E E of pebbly to cobbly gravel. East of Big Thompson Canyon to South Platte _\ 77 4' El River, overbank sediments are dominantly sand, grading downstream to I 44// \ 2 W . . _-~—' D n. g dominantly Silt. GPEELEV—LOVELAN Q ‘ / Fme stipple indicates areas of widespread deposition of pebbly to cobbly gravel; includes some small deposits of pebbly sand and cobbly to bouldery gravel. Coarse stipple indicates widespread deposition of cobbly to boul- dery gravel; includes some small deposits of pebbly to cobbly gravel Local areas of intense lateral scour fl 7A WV E: /_. / - Extensive channel scour within floodways of Big Thompson River and North ' l .4 Fork; deposition of cobble to boulder gravel. Bouldery gravel bars com- 4945 j 1 7; / was TERN ‘ . mon, especially along insides of bends. Overbank sands to boulder gravels ’ . ‘ }- Comm _ 1L I r _ . occur locally. Colored areas represent approximate extent of flooding. Fine ’ ’ '. stipple indicates areas of widespread deposition of cobbly to bouldery \/ ' T gravel; includes some small deposits of pebbly to cobbly gravel. Coarse T» 5 N- \—: 7‘ " stipple indicates widespread deposition of bouldery gravel; includes some on proportion to sand downstream except locally near LL “ . small deposits of cobbly to bouldery gravel (Ll bank edge and in subzone of coarse to medium sand 1 . . . Lu nea r_west edge of zone; much mica except in subzone. ,, ) Intensive channel scour throughout floodway of Big Thompson River; local I; Lei/LbaXLTnIUfrtnt thIIckmtafis decreassed downsglream from , deposition of cobbly to bouldery gravel. Much of the sandy to bouldery : Whig obs :rvg‘sjs an 6 'n' tructure '5 at bedd'"9 ’ ‘ ) , material scoured from the channel was deposited farther downstream along 2 ’ I ‘\_ ' ' reaches with lower gradients. Overbank sands to boulder gravels occur 0 ' locally. Colored areas represent approximate extent of flooding. Coarse 8 ‘ stipple indicates local deposition of bouldery gravel D l g y W Segments of highway severely eroded or destroyed by floodwater ,. 13—44. g . I o 3 Boulder-measurement site—Number after letter B corresponds to river 0 g E mileage of site. Sizes of largest boulders are summarized in tables 7 and 8 in :L ‘l . text LL . 31,200 ft3/s . . . . I o Peak—discharge measurements (In cubic feet per second) by Water Resources :I; Division of the US. Geological Survey , 'lL —-———- 4 lsohyets (in inches) for July 31—August 2, 1976 storm, slightly modified from 77 ll unpublished data provided by the National Weather Service I l y ' lI #l———i— Mileage marker—Indicates distance in miles along riverbed I L 1 _ \\ I 6.5 6.0 4A.: p m ' v ._ __ I J. ' \ \7 I D. M ° - ““931 Fr" ' I91 W;\ K-A [/l(j 1‘? pm _,_/ k7 I, \v a; I . 40°22'30” - \ \\ N\\ \j M (u/Ly > REELEY s 3 MI a I 105°02’30” RBBW . v _/-) 47r30uG - 104 45° I H 2.7 MI TO COLO. 60 W /o' I - 4O 22 3O / 11.2. I lI I \ L. L‘ ‘l ‘ Falrview c :‘ \44 . 3 \l ‘ . “I II - \ . n ' ' a - l . dill) 754V“. '11- ., 4:! V§E§%€%§Z\E/4fi%gi TSN f Overbank-sedimentation zone 4: Silt; local fine sand, some- coarser, near bank edge; mica abundant. Maximum thicknessgenerally|essthan6in.,common|ylessthan 1 in.; areas of no sediment occur within areas of 1 LOVELAND 8 MI. 14 MI, TO us. 287 . sedimentation ' \ l I 'I 'i \ / \J nit/I1 K r I. ll 4 v J1 21M” ” l l ‘ J/l RC5, ‘. 7 7M T.5N. L \\ u? X Q X “75 TAN. A a’ f ‘\ T. 4 N. 4669 2;” 03 SEES-5': LL: ,7 .,7 _. j ' L. ‘ ‘ L '1 II. / L‘: ' L‘»_.,4-"' L 1,? I L >L T L , '- 7/7,” 1' ‘ TL L I . SJ 4.0“ 20' N“ At t ————— - " -____ ____: , " v - ______ ' .. . ‘ _. 1 ' ' ‘1 o r \‘r‘ul .. . ‘ ' ' " I ——————— l ./ . V _ _ , ”7. t _ __________ 105 00 - * n . 7 u I 5230” R.B7 W. 50' RIEGW. 47'30” 104:1405’20 Base from US. Geological Survey, C. BIG THOMPSON RIVER, EAST OF LOVELAND TO CONFLUENCE WITH THE SOUTH PLATTE RIVER . Drake, 1962 Johnstown, 1950, photorevised 1969; _ Flood effects east of the mountaIn front _ Loveland, 1960, photorevised 1969; Masonville, SCALE 1,24 000 mapped by E. J. Crosby, 1976. Flood effect In . 1962, photorevised 1971; Milliken, 1950, photorevised 1 V 0‘ . COLORADO BIg Thompson Canyon mapped by P. W. SchmIdt 1969; and Windsor, 1950, photorevised 1969 ,_. r 2 = 1, H 1 Mil-E and R. R. Shroba, 1975. 1 5 o 1 KlLOMETER CONTOUR INTERVAL 10 AND 40 FEET DOTTED LINES REPRESENT 20-FOOT CONTOURS NATIONAL GEODETIC VERTICAL DATUM OF 1929 APPROXlMATE MEAN DECLlNATlON 1979 QU.S. GOVERNMENT PRINTING OFFICE: l979—677—026/4 GEOLOGIC AND GEOMORPHIC EFFECTS OF THE 1976 STORM AND FLOOD, BIG THOMPSON RIVER AREA, COLORADO UNITED STATES DEPARTMENT OF THE INTERIOR PROFESSIONAL PAPER 1115 GEOLOGICAL SURVEY PLATE 3 Altitude (FeeTI 7.600 _/OLYMPUS DAM DRIY GULCH LOVELAND HEIGHTS - DEBRIS FAN UNNAMED GULCH 7.400 — DEBRIS FAN I GLEN COMFORT DARK GULCH '8 DEBR'SFAN I NOELS DRAw 3.6 - I 7,200 —‘ . 2.2 LONG GULCH\ MAJOR LATERAL CUTTING AT RABBIT GULCH e————LOCAL REWORKING 0F SANDY TO PEBBLY ALLUVIUM OUTS'DES 0': BEND MAJOR OVERBANK I 7‘000 _ MAJOCRUIf/SAITDEERSAIéchETJrIJIéG AT, DEPOSITION I I SANDY TO COBBLY CHANNEL FILL p SANDY TO PEBBLY CHANNEL FILL SIEIIIKYNJEOLCF‘IEELY. 6.800 — BOULDER CREEK MOSTLY S I L V E R P L U M E G R AN I T E GRANODIORITE MICACEOUS SCHIST I I I I I I I I 6’600 River Mile 5? 56 55 54 53 52 SI 50 7900 ‘— LONG GULCH 6.800 —"\MAJOR SIDE CUTTING V72 \ TRUE IGULCH WALTONIA SULLIVAN GULCH 6.600 a<— SANDY TO COBBLY CHANNEL FILL—> \ I I (UNNAMED GULCH I I I? 6.400 —' DRAKE (INTERSECTION ROUTES 34 Si IO3) TWBOULDERY GRAVEL BARS—T I NORTH FORK BIG THOMPSON RIVER 6.200 — I I 4-7 MIDWAY BOULDERY GRAVEL BARS DAM | EINTENSIVE SCOUR —><——EXTENSIVE SCOUR—Q EXTENSIVE SCOUR PEBBLY TO COBBLY I 7.4 s 000 — EISXELREEI‘I’ISJJEI’ COVERED WAGON ' 'NTENS'VE SCOUR STRAIGHT FLOODWAY LATERAL SCOUR,ESPECIALLY RESTAURANT AREA COBBLY To BOULDERY ALONG OUTSIDES OF BENDS w BOULDERY 5,800 — GRAVEL BAR NARROW AND CONFINED 5.600—— GNEISS AND SCHIST ”MW“ ’ <—-——EXTENSIVE SCOUR INTRUDED BY MANY LARGE MOSTLY MICACEOUS SCHIST BODIES OF PEGMATITIE | FAULT ZIONE IGNEISS AND SCHISJT I I I 5’400 49 48 47 46 45 44 43 42 LOVELAND POWERPLANT 5‘8” I CEDAR CREEK \fl I GROUSE HOLLOW . !/ DICKSON GULCH _ \ | 5500 II 2‘4 I I-—THE NARRows———I EXTENSIVE SCOUR ALONG _. NARRow AND EEOSIIIIXY WIDESPREAD DEPOSITION 0F PEBBLY MOUTH OF BIG THOMPSON CANYON SAND IN BROAD FLOODWAY,LOCAL 5,400 _ SCOUR ALONG MEANDERING CHANNEL W, I DAM I-—BROAD FLOODWAY- | CHANNEL SCOUR e. FILL BOULDERY-COBBLY BEDLOAD DEPOSIT I I GREEN RIDGE GLADE LARGE BOULDERY GRAVEL AT UPSTREAM END OF BROAD FLOODWAY I I BUCKHORN BAR CREEK -<——INTENSIVE SCOUR WITHIN NARROW AND 5200 _ CONFINED FLOODWAY I PALEOZOIC ROCKS I "—"‘— EXTENSIVE LATERAL CUTTING OF CHANNEL BANKS AND WIDESPREAD OVERBANK DEPOSITION OF 5.000 — SAND AND SILT AND LOCALLY COARSER MATERIAL \ FAULT GNEISS AND FA U LT ZONE GNEISS AND SCHIST GRAN|TE PALEOZOIC AND MESOZOIC ROCKS ZONE SCHIST I I I I I I I I 4 . ’800 4| 4O 39 38 37 36 35 34 DRY CREEK I TAFT AVENUE U-S-ROUTE . 287 I I 5,000 — I EXTENSIVE LATERAL CUTTING OF CHANNEL BANKS AND WIDESPREAD OVERBANK DEPOSITION OF SAND AND SILT AND LOCALLY COARSER MATERIAL 4,800 —- RIVER AT GRADE—UNDERLAIN BY PALEOZOIC AND MESOZOIC ROCKS AND UNCO,NSOLIDATED QUATERNARY DEPOSITS 33 32 3| 3O 29 28 27 26 River Mile A. Profile of The Big Thompson River from Olympus Dam To U. S. Highway 28? showing generalized bedrock conditions and principal flood effects AIIitude (Feet) 7,800 — 7,600 “- GLEN HAVEN e VIDENCE OF SIG- NNIIEICANT EROSION FOX CREEK DEVILS GULCH 7,400 — OR DEPOSITION —— ' . PIPER MEADOWS DRAINAGE 7.200— I DUNRAVEN GLADE REWORKING OF CHANNEL ALLUVIUM AND EXPLANATION OVERBANK DEPOSITION OF SANDY DEBRIS . 7.2—Average Intermediate diameter.in 0/— feel,of largest boulders transported I/COBBLY GRAVEL . MILLER FORK by the flood (tables 7 and s) 7 G R A N IT E—+ W I .000 — GNEISS AND SCHIST . . I I -<———EXTENSIVE SCOUR ALONG NARROW I PEBBLY SAND AND CONFINED FLOODWAY / ALLUVIAL FAN River Mile IO 9 "\ —6.800 ' <——WIDESPREAD DEPOSITION OF PEBBLY . SAND IN BROAD FLOODWAY,LOCAL 7.800 _ D E V I L S G U L C H SCOUR ALONG MEANDERING CHANNEL_¢—>— H6,600 , /PEBBLY SAND GALUCHIE -<—— INTENSIVE SCOUR ALONG ALLUVIAL FAN GUI—CH BOBCAT NARROW AND CONFINED 7.600 L FLOODWAY——- PEBBLY SAND I GUI—CH ALLUVIAL FAN WEST CREEK I 6, 00 EXTENSIVE SCOUR ALONG I STATE I NORTH FORK INJRIIEDTIEOPSSTBE. F'SH - DRAKE 3 TI ' 7,400 — COBBLE~GRAVEL BARS—’- HATCHERY GLEN HAVEN . —6.200 «—~——WIDESPREAD DEPOSITION OF PEBBLY SAND IN BROAD FLOODWAY; ECOUR LIMITED To LOCAL AREAS OF CONFINED CHANNEL AND OUTSIDE OF BENDS M 7,200 — .-<—|NTENSIVE SCOUR WIPESPRYEASDNDEPOSITION OF IN TGRNUEDIESD 3 BYA ND SC HI 5 T E EBBL AD———>- __ MANY LARG _ 6,000 ‘— GNEISSANDSCH'ST GNEISS AND SCHIST BODIES OF PEGMATITE FAULT ZONE SILVER PLUME GRANITE 7,000 I I I I I I I , I I I , _ 2 I O 8 7 6 5 4 3 2 I River Mile 0 5. Profile of Devils Gulch and NorTh Fork Big Thompson River from Glen Haven To The confluence wiTh The Big Thompson showing generalized bedrock condiTIons and principal flood effecTs Stream profiles prepared by P. W. Schmidt Vertical exaggeration '0 X Geology of bedrock and surficial deposits mod- ified from Braddock,Calvert,and others(1970I, STREAM PROFILES OF BIG THOMPSON RIVER AND NORTH FORK IIIIIA.~II:IIIIAI.W Flood effects by R. R. Shroba, P. W. Schmidt, and BIG THOMPSON RIVER, COLORADO “my